When your Ice Trap or Snake Trap are triggered you entrap all afflicted targets, preventing them from moving for 4 sec.
Entrapment is a passive hunter ability learned at level 55 for hunters who have chosen the Survival specialization. Note that this only triggers with [Ice Trap] and [Snake Trap].
Patch 3.1.0 (14-Apr-2009): Entrapment redesigned: When your Frost Trap, Immolation Trap, Explosive Trap and Snake Trap are triggered, you entrap all afflicted targets, preventing them from moving for 2/3/4 seconds.
Patch 2.1.2 (2007-06-19): This talent will no longer causes Hunter to stand up or stop eating and drinking when it procs.
Patch 2.1.0 (22-May-2007): Effect duration reduced to 4 seconds and is now subject to diminishing returns in PvP.; The tooltip has been corrected to indicate it works on Snake Trap.