Haste Runes are hero runes which, when activated, increase a random enemy creature's wait time by 1.
This rune comes in a variety of rarities; however, it is recommended that you only level up the 5 star version, since it scales the quickest. Even the 4 star version isn't worth leveling, considering that it can only reach a maximum of a 60% chance to proc. The 5 star version, though, can reach a maximum of 100% (increasing by 10% per level).
A level 5+ 5 star Haste Rune can greatly help in gaining and keeping Board Control while in battle, so make sure that you use it if you have one lying around!