Here's the nice easy definition/explanation. It gets a lot more complicated, if you get into it.
Predeterminism is the idea that every event has a cause which is necessitated to be that cause (and to happen at that time), because some operating principle required it to happen. (It's like determinism, but with an overarching guiding principle).
All kinds of crazy reasons. Here's one of the simpler ones.
1. God is omniscient.
2. Therefore, God knows, and has always known, when you will die.
3. Therefore, The time that you are going to die has been fixed for as long as God has been omniscient.
4. This can be generalized to any event (i.e. God knows, and has always known, X).
Fun huh? Both determinists and predeterminists usually believe that given enough information a person could predict the future with supreme certitude. The difference is subtle, but generally speaking determinism is limited to the idea that every event follows causal laws. Predeterminism adds some other factor, one that is, in some sense, omniscient and/or overarching, setting down not only the rules, but also the form those rules must take. (Predeterminism gives you the rule book and stacks the deck, so to speak).
Another way of looking at it is that determinism is a chain of causal statements; 'a happens, which causes b, which causes c, etc.' While c has been determined to happen ever since a came about, if for some reason a or b did not happen, then c wouldn't be caused, and so wouldn't happen. With predeterminism, 'a will happen, and b will happen (etc.)', but the cause is God (or something else of that sort). If a doesn't happen, that doesn't really have any bearing on weather or not b happens; we have the same reasons for believing b will happen, weather or not a comes about.
If you like this, you could go even further, in to fatalism.
The popular usage of the term 'predeterminism' seems to be just any form of the idea that the future is already set.
You might want to browse through these:
, Soft determinism
(beware, this word has multiple meanings).