Nikola Tesla attracts both serious historical interest and exaggerated stories. The best way to appreciate him is to separate documented engineering from later legend.
Where the real contribution shines
Tesla’s work on alternating-current motors, polyphase systems and high-frequency electrical experiments helped shape the development of practical electrical power. AC systems are especially useful because transformers make it practical to change voltage levels for transmission and distribution.
Why high voltage looks magical
High-frequency, high-voltage demonstrations can create spectacular sparks and illuminate lamps without ordinary wired connections. They can look like evidence of energy appearing from nowhere. They are not. The apparatus is receiving power, converting it and coupling energy through electric and magnetic fields.
A better question than “free energy?”
Ask where the input power comes from, what the output power is, how long the effect can be sustained and how much energy is lost as heat, radiation or other forms. Tesla’s real work rewards measurement. It does not need claims that violate conservation of energy to be fascinating.