Recall is a ability unique to the Protoss Arbiter. Recall creates a kind of warp field where a group of friendly units is instantly teleported to the Arbiter's location. You can use recall to bring forces to the front of the battle or back to the safety of the proton cannons back at your base.
Arbiter: Recall
The Arbiter teleports a group of friendly units in a 5 x 5 area to its location. Note that burrowed zerg units are immune to this ability, but siege tanks in siege mode can be recalled.