To move the troops forward. Especially as dismounted troopers. This is the old ADVANCE from Scott's Infantry Tactics.
First time through, the undertone command is Battalion, Forward!
Second time through, the command is March! and step off on your left foot.
The reason this call is different from the Infantry Call is to retain independent bugle signals...even if Infantry Skirmishers and Cavalry Foragers are working in the same area. Used as well for Column and Battle Line advances at a walk.