The belief that if one knew absolutely every single thing there is to know about the present, than that person could predict with perfect accuracy every single event that would happen in the future. This is based off the idea of cause and affect, and the structured and unchanging laws of physics. Simply put, determinism = fate.
This is often related to theology in that unless by some unknown means time has existed for all eternity, something had to have started existence. Has all matter simply been here forever, with no beginning? That's a pretty hard to believe theory, if you think about it. Something HAD to have started the universe somewhere, sometime. And with the first single event that started the chain reaction that is time, the fate of everything that has and will ever happen has already been written. But then brings the question, from where did the creator of time and space come from? But meh, these are things we will probably never know, I think you get my point anyway.
Example of determinism-
Knowing the velocity, direction, wind speed, and exactly every other factor effecting a rocket, you can predict perfectly where it will land. Why couldn't you? The same goes for absolutely anything else in the world. If you knew absolutely every factor effecting a situation, you could perfectly its outcome with complete accuracy.