Haste is a keyword ability that lets a creature ignore summoning sickness. Normally, a creature can't attack or use an ability that requires the tap symbol ({T}) unless it has been under your control since the start of your most recent turn. A creature with haste can attack as soon as it comes under your control — and if the reminder text says so, it can also tap for an ability right away.
In play, haste turns a creature into an immediate threat. Your opponent can't rely on a turn of safety after you cast it, and surprise attackers can force blocks or deal damage out of nowhere. It also lets tap abilities fire off the turn they arrive, making mana dorks, pingers, and combo pieces far more explosive.
