When you kill a monster, you need to wait some time untill this monster is generated by the server. Players will refeer to respawn as the delay that you have to wait until the monster reappears, the fact of the monster reappearing, or the place where some monster respawns.
The time of respawn will take longer when your server has fewer people online. An advantage of hunting when many are online is the speed the monsters will respawn, thus giving you more experience. However, with more people online there is a better chance that your hunting spot will be claimed or public spots will be crowded.
Some tips for a better respawning
- Do not stay at the place where monsters have to respawn, player presence interrupts respawning (go some squares away and wait there fishing, eating...).