Dash attack is an attack performed by pressing the attack button while dashing, (running, in other words). To do a dash attack immediately after starting a dash, press the c-stick down instead.
Dash attacks typically use the momentum behind the dash to pull off some sort of attack, like a sliding kick. Most dash attacks come out quickly, but have noticeable ending lag and are predictable, making them very easily shield-grabbed. In Brawl, certain Dash attacks can be interrupted by performing an Up Smash, which is known as Dash Attack Canceling. Often, a combo is used with the low, more combo based knockback of the dash attack, leading up to the more powerful KO move Up Smash. The Dash Attack can also be cancelled by the Grab Cancel.