Much as the name indicates, respawn means to spawn again. This generally refers to the time needed for some enemy unit or player to be resurrected after death.
In Guild Wars, unlike in many other MMORPGs, monsters in an instance stay dead once killed. However, if a player leaves and re-enters the instance, he/she obtains a fresh batch of monsters to kill. This is sometimes referred to as "respawning", though it is technically simply refreshing the instance. There are very few cases — Chained Souls in the Underworld, for example — of monsters that are automatically brought back to life every few minutes.
Farming in Guild Wars generally requires running between linked areas and towns. A player cannot simply stand in a single spot and kill respawning creatures.