What is Tesla's cosmic radiant energy?

Exactly, radiant energy relates to the following 2 events:

In 1889 Tesla began experimenting with capacitors charged to high voltages and discharged in very short time intervals. These very short pulses produced very sharp shockwaves which he felt across the front of his whole body. He was aware that closing a switch on a high-voltage dynamo often produced a stinging shock. This was believed to be static electricity and it occurred only at switch-on and only for a few milliseconds. However, in those few milliseconds, bluish needles of energy stand out from the electrical cables and they leak to ground, often through the bodies of any people standing nearby, causing immediate death if the installation is large. While the generators of that time were rated at some thousands of volts, these discharges were millions of volts in intensity. The generator problem was eliminated by the used of highly insulated switches which were provided with a very large ground connection.
Tesla searched the literature to find references to this radiant energy but he could not find much. In 1842, Dr. Joseph Henry had observed that steel needles were magnetised by a Leyden Jar spark discharge located on a different floor of the building. The magnetising wave had passed through brick walls, oak doors, heavy stone and iron flooring and tin ceilings to reach the needles located in a vault in the cellar. In 1872, Elihu Thomson took a large Ruhmkorrf Spark Coil, attached one pole of the coil to a cold-water pipe and the other pole to a metal table top. This resulted in a series of massive sparks which electrified the metal door knob of the room and produced the stinging shockwaves which Tesla was investigating. He found that any insulated metal object anywhere in the building would produce long continuous white sparks discharging to ground. This discovery was written up briefly in the Scientific American journal later that year.

Some articles understand that radiant energy is wrong, they argue that it is an energy form of electromagnetic waves. Incorrect.
Radiant energy, not electricity, not words, and not a form of electromagnetic wave.
Radiant energy, it is part of the Ether doctrine, you can understand, it is part of the Ether. Tesla mentioned this energy related to "zero point" energy.
Radiant energy, is a form of radiation, it is considered a conductor of electricity, but there is no current flowing.
With radiant energy, electricity is generated, broadcast, and captured and put to use wherever it is useful. The most widely used method today uses the sun’s energy generation and captures it with solar panels. Tesla’s method didn’t use visible light, and it didn’t require line of sight, it uses frequencies that pass through most obstacles easily. His public demonstrations used standard electric generators, broadcast that power, captured and used it on the stage. His first patents for this method are Apparatus for the Utilization of Radiant Energy and Method of Utilizing Radiant Energy.

Image of Radiant energy generated by electrical impulse:

This wave will only occur if the current pulse is in one direction, i.e. it will not occur if the wire is fed with alternating current. The wave radiates out horizontally from the vertical wire in every direction in the form of a shock wave. It is a one-off event and does not repeat if the current in the wire is maintained. The Radiant Energy briefly unbalances the zero-point energy field and that causes an energy flow as the field moves back into equilibrium again. A tiny fraction of this massive, brief energy flow can be picked up and that collected energy is more than 100 times greater than the energy needed to generate the spark which triggered the energy flow in the first place. This is the energy which the tube was designed to collect. Consequently, the tubes are fed with a train of high-intensity, short-duration, DC pulses to generate repeated waves of Radiant Energy. It is the pick-up of the resulting excess energy which allows the motors run without the need for the batteries to be charged by any conventional source of current.
How Does Radiant Energy Work?
In a word: inefficiently. Tesla’s original plan was to use standard electrical generation and broadcast it worldwide. This has a fatal flaw, in that electrical generation isn’t free, but the use of it would be.
There is a light at the end of this tunnel, however. In 1933, Tesla announced that he had discovered a method of capturing part of the sun’s non-visible energy for use. He called the frequency he was using ‘cosmic rays’, and claimed that it would work as well at night as during the day. In this newspaper interview, he stated that the method would be ready for commercial development in the near future.
Unfortunately, he never patented the process, and he did not find a commercial backer for developing it. When he died in 1943, the Office of Alien Property collected his notes, which were held as classified until they were delivered to his nephew in 1952. They are currently on display at the Nikola Tesla Museum (Muzej Nikole Tesle) in Belgrade, Serbia. They do not contain information on the mechanism he used or expected to use to capture these cosmic rays.
Tesla withdrew from public view in the 1930s, and there was very little public research in his work for over 30 years after his death. After the energy crisis of the 1970s, interest in his work resurged, and many researchers are now trying to re-discover his method of using cosmic rays as a power source.
Do we know how to exploit cosmic radiation?
The reality is yes. Many people succeeded, based on Tesla's remaining works.
Here are some typical evidence:

Want to know why generators have not been used to replace the current electricity industry? To answer this issue, see the answer to the question: Why is the power generation by means of magnetic propulsion not more widespread?

Revealed At Last: Ancient Invention Generates Energy-On-Demand