The basic idea is you put a rat in a cage and give him electric shocks. Each shock is preceeded by a light in the cage and if the rat runs over and hits a lever within, say, 3 seconds, no shock is delivered. Once you train rats on this sort of thing they get very good at it. They almost never receive shocks anymore and for the most part, they're just like normal rats. After a while though, you change the experiment and 'disconnect' the lever. The lever now does nothing. The light comes on, the shock is delivered and there's nothing the rat can do to stop it. At first they press the lever feverishly trying to fend off the shock but after a while they just give up and lie there in a heap. Even if you reconnect the lever, it does no good. The rats are still lumps afterwards. It's as if they've learned there's no point trying anymore since nothing they do can help themselves. They're just fucked.
I can relate.