It's the Flurry Rush Action. it's a special action, not an attack action.

It is a mini-dash because I have personal experience witnessing martials constantly being unable to close the distance to targets and when that happens it sucks. It also helps make it relevant even in lower tier play when two weapon fighting and this feet would give you the same number of attacks. It only gives you more attacks once you reach 9th level.

You only get 3 uses of the ability every day, so it is limited in how often you can use it. You could spam it 3 times in one combat, but that's like if your wizard blew all their highest level spells in one shot. It's devastating damage wise, but since there's a utility component in it's movement ability and you don't know what you'll be up against later, using them all at once might not be the best move.