It's a turn based strategy game, so you're trying to build cities, produce military units, research new technologies, and many other things. At the higher difficulty levels the AI is already pretty good at doing those things on it's own, but it also gets some large bonuses that enables other civilizations to outpace you by quite a bit unless you really know what you're doing. Even then, getting a bad starting location or researching the wrong technology early in the game often means you lose badly.