Bloodthirst is a creature mechanic that rewards you for damaging an opponent before casting your creatures. A creature with bloodthirst X enters with X +1/+1 counters if an opponent has already been dealt damage that turn.
Specific examples include Scab-Clan Mauler and Skarrgan Pit-Skulk growing through bloodthirst, while Bloodlord of Vaasgoth can give all other Vampire creature spells bloodthirst 3.
