Literally translated as “before,” it is more widely taken as “initiative” as in taking the vor/initiative.
This may mean attacking first (and so forcing the opponent to defend), but can also be taken to mean any method by which you, before the opponent has time to act, eliminate options for them or make their attack more predictable (eg. manoeuvring them into a corner).
The correct action in taking the vor can be an overall strategy (for example, you know your opponent is a highly offensive fighter, aim to create an opening to lure him in), or it can be an immediate tactic (you see him change guard so immediately take the vor with a meisterhau).
Some people accord almost mystical significance to the Vor and the Nach; don't - it's fight theory 101.