Haste is a static ability that allows creatures to ignore the rules informally known as "summoning sickness". This means that they can attack, and activate tap or untap abilities on the same turn they enter the battlefield.
Haste first appeared on Nether Shadow in Alpha, although it did not have a keyword at the time and was simply spelled out in the rules text.
Example
Aidan casts a Chandra's Phoenix and moves to combat. Normally he wouldn't be able to attack with it it, because of the summoning sickness rules. However, because the phoenix has haste, he can attack with it right away.
Comprehensive Rules
702.10. Haste
- 702.10a. Haste is a static ability.
- 702.10b. If a creature has haste, it can attack even if it hasn't been controlled by its controller continuously since his or her most recent turn began.
- 702.10c. If a creature has haste, its controller can activate its activated abilities whose cost includes the tap symbol or the untap symbol even if that creature hasn't been controlled by that player continuously since his or her most recent turn began.
- 702.10d. Multiple instances of haste on the same creature are redundant.