Action Skills are powerful abilities that are located at the top of each Vault Hunter's Skill Tree. Each Vault Hunter has a unique Action Skill, and these are often a big reason for chosing to play that particular Vault Hunter to begin with. Before you can use your Action Skill, you will need to unlock it, which can be done at level 2. Just head to the Skill Tree and purchase the skill (you can't go wrong, since you can't choose any other skills yet). This will make it active and ready for use.
The botton for using your Action Skill in Borderlands 3 is the following:
Xbox One: LB.
Most Action Skills in the game have a cooldown, so you can't spam your Action Skill all the time. The cooldown counter is located at the bottom of your screen, and the Action Skill icon will glow when it is ready to be used again.