A prefab is a pre-built THING that you can simply and easily drag out of a prefab library and toss into your map. Quite a time saver, esp with crates! There's tons of pre-fabs for all-sorts floating around the net, and its easy to make your own by selecting some brushes, entities (whatever!) and clicking the "create prefab button"
How do you find tons of pre-built prefabs?
DO A GOOGLE!!!(clicky)Im sure you'll find the right car/truck somewhere!