Tesla understood a few things
His discovery of what we now call the 'Schumann Resonance' and the standing waves contained within it led him to the wireless transmission of electricity.
During those experiements he found he could 'pump energy into' and 'extract energy from' that resonant cavity formed between the earth's surface and the ionosphere
Due to the solar wind's constant charging of our magnetosphere and thus trickling down to the ionosphere with several GW of power continuously Tesla found he could extract slightly more energy that he had 'pumped in'
Thus the 'free energy'
Not free and not seriously significant in todays modern world in terms of global power consumption.
These days they explain the wireless transmission of power by invoking 'Zennek waves' which is an explanation of how power is transmitted in a waveguide by flowing on the surface of the waveguide.
This is necessary due to the fact that resonant waves in a waveguide are not sensible by normal methods see TE, TM & TEM modes of transmission in a waveguide and this is why Wardenclyffe doesn't look like a normal antenna.