Fast Flo is a boost that will make Flo move more quickly in a level. Instead of walking at her normal pace, Flo will run around the restaurant and jump over tables if necessary. This is especially useful when you have very impatient customers!
To use the boost in a level, simply purchase it by clicking on the icon on the level intro page. Each level will have 3 boosts available to be purchased and used in that level. Boosts will automatically activate when the level starts, and they'll last for the duration of the level!