Upon taking this feat your Action Surge ability is improved. You now have access to a second use of the Action Surge ability (On Your turn, you can take one additional Action on top of your regular Action and a possible bonus Action.) And subsequently at 17th level you now gain a 3rd use where as normally it would be the second.

 Starting at 8th level, you can push yourself beyond your normal limits for a moment. On your turn, you can take one additional action.

Once you use this feature, you must finish a short or long rest before you can use it again. Starting at 17th level, you can use it 3 times before a rest, but only once on the same turn.