Your pet growls at the target, generating threat and taunting the target to attack the pet.
Growl is a pet ability shared by all hunter pets. It is a special form of taunt, raising pet on the aggro list by generating a high amount of pure threat. Unlike taunt, it does not force the target to attack the pet for any duration.
Patch 6.0.2 (14-October-2014): Growl now has a 30-yard range.
Patch 1.7.0 (13-Sep-2005): To help make "hunting" pets more viable in more situations, the Growl ability is now free to all pets.