While your pet is active, you and your pet will regenerate 1/2% of total health every 10 sec., and increases healing done to you and your pet by 5/10%.
"After the hunt, we rest here as one entity and together feel our strength return.[1]
Spirit Bond is a Beast Mastery talent for hunters. Every ten seconds, the hunter gains 1% of their max health per rank, up to two ranks. The talent is constantly active as long as the hunter's pet is alive. The healing effect also applies to the pet.
It seems to generate some threat on your pet. (You shouldn't have taken any damage for the tic to affect you, thus removing the possibility of gaining any aggro from this buff.)
Trivia
Originally this healed you for a small amount when your pet hit the target, and it lasted only 30 seconds.
Patch changes
Patch 5.0.4 (2012-08-28): No longer increases healing done to you and your pet.