Haste is an enchant used on leggings. Haste gives you the haste effect, just like how a beacon gives haste in vanilla minecraft. This type of effect enables you to mine so much faster.
Haste I gives the effect of Haste VII. Haste II gives the effect of Haste IX.
Haste is obtained just like how any enchanted books are acquired; through mining, fishing, farming and cutting down trees.