Hunting zombies is a technique that players use to quiclky win a round. It usually refers to the action of going to the spawn (or wait there until the zombie spawns) to corner and/or kill the spawning zombies without allowing them to infect anyone, forcing humans to get a quick victory without reaching the map's end.
Hunting itself is usually not allowed and can get you kicked or banned from the server (depending on their rules, some servers don't have anything against hunters). However most zombie spawning settigs counter the idea of hunting by default, spawning enough zombies so they can't be easily cornered if they get teleported to the spawn; or by spawning the zombie/s in the middle of the crowd allowing them to grow quickly in numbers to the point of hunting not being viable.
Sometimes hunting is allowed and even enforced when there is a case of fail zombie/s. This usually happens if the zombie was AFK, not paying atention, unskilled or just plain unlucky leaving him alone without the needed fast first infections. In those scenarios is usually "mandatory" to kill the zombie instead of wasting a whole round worth of time for a round with no action whatsoever. But when enough zombies are available for some action on the round, remember that Zombie Escape is about escaping zombies.
Anyway; both the rules and the effects of hunting vary from server to server depending on their settings, and also on maps depending how they were made. Settings like strong zombies (high HP/Speed and low knockback), respawning zombies (after diying as a zombie, sometimes even after joining late or diying as a human), or spawning multiple zombies (instead of just one) discourage hunting and can even make it virtually imposible on some situations.