This post contains major plot spoilers for Neon Genesis Evangelion.
Neon Genesis Evangelion is one of the biggest and most influential anime series ever created, every episode shifting between philosophical introspection and giant-scale robot-vs-monster fights. The robots themselves, called Evangelions, or EVAs, appear as huge mechanical humanoids that must be piloted by a person inside a cockpit in the body of the robot, who "syncs" neurally with the EVA's brain to wield whatever weapons it has. Swords, guns, whatever. Seems pretty straightforward from here, but if you've done any googling about the show, or seen the first episode, or even finished the series and are still confused, you might be wondering: What IS an EVA, anyway?
I'm gonna start off by saying if you haven't seen the show yet and are planning to watch, DON'T go on the wiki and try to look stuff up, because you will be spoiled, and also confused. (That's nothing against wikis, they're very helpful, but be careful how you use them!!) Like any machine, there are rules of operation. The EVAs can't stray far from NERV Headquarters in Tokyo-3, as each is attached to a power source deep in the ground by an umbilical cord, which is plugged into the EVA's back and can generate its power indefinitely. Once the cord is unplugged or severed, the EVA has power for only a few minutes before it completely shuts down.
The EVAs' human pilots are shunted into them through an Entry Plug, where they are suspended in LCL fluid, a weird orange juice you can breathe like air. They also have to be piloted by certain people -- children who have been born after the Second Impact, and each EVA seems particularly attuned to only one specific person. Only Shinji can pilot Unit-01, only Rei can pilot Unit-00, and only Asuka can pilot Unit-02. Otherwise, the EVA's sync rates, shown in waveforms, will be completely mismatched.
If you've gotten far enough to see the conclusion of the second episode of the show, you've seen enough to know that the EVAs are not machines, not entirely. Shinji agrees to pilot EVA Unit-01, but, due to inexperience, fails miserably. Unit-01 is severely damaged by the Angel Sachiel; its arm is broken, and an eye is pierced before it loses power and Shinji faints from the phantom pain. Shinji can't remember anything, but in a flashback, we see the EVA come back to life on its own, without power, and viciously attack and destroy the Angel. Its damaged metal helmet falls off and Shinji watches it regenerate an eye -- a real, flesh and blood eye -- and he screams and passes out.