Boost (Mount) is a skill available on some mounts. It allows a 100% speed boost to your character while mounted!
However, for your mount to be faster, you've got to feed it some food to make it energetic! Purchase Food For Mounts
from the Mount Manager NPC. The first place you'll probably run into him is in the city of Prontera, to the east of the Prontera North Gate
There, Taxi will be waiting for you to sell you (and your mount) some delicious Food for Mounts
for 1 Zeny each. Each cast of Boost (Mount) will consume 1 Food for Mounts. Once you use your Boost (Mount) skill, you'll be zooming across the land!