its when both sides of a system are at the same temp. for example (sorry is this is a bad example) one of your hand was really cold and one was really hot from running it under hot water and you clamp your hands together the molecules in the hot hand (which are moving much faster due to the heat, will start bumping into the slow moving molecules in the cold hand, until it all balances out and both hands are the same temperature.
I hope the helps :)