Haste is a spell-granted ability that allows the unit to move more quickly across any terrain. When moving on the world map or in combat, the unit requires 1 fewer movement point than normal for each space they move to, regardless of terrain type.
The effect of Haste is thus more prominent for methods of movement that were already very efficient to begin with. For example, grounded units traveling by road will only require 2 movement points per space instead of 3, meaning they can travel about 50% farther in a single turn (similar to, but not stacking with, the effect of Enchanted Roads), while a unit that is tunneling underground will require 9 movement points per space instead of 10, which won't impact the maximum number of spaces that can be tunneled through in a single turn, even for the Goblin Big Beetle.