When attacked from melee range, bosses with this property will sometimes teleport to a random location when their hit points drop below 33%. The monster will partially heal when it teleports.
This property was very difficult to deal with in early versions of Diablo 2, since it let monsters regain their full hit points each time they teleported. Battles between low damage characters and high hit point teleporting monsters could go on almost indefinitely. This property was nerfed in subsequent patches and in v1.09+ monsters teleport much less often, and heal only slightly when they do.
Some monsters, such as Baal and Uber Izual, will teleport without regaining hit points.
This attribute gives the monster a new behavior: it will Teleport (or try to) when below 33% health. Ranged monsters will Teleport out of range when an enemy closes in to melee range. Basically, this attribute makes them run away to either heal or attack from range.