Action Type refers to what occurs when you pull the trigger.
With a single action firearm (revolver or pistol), the trigger simply releases the hammer or striker. With a double action firearm, the trigger cocks AND releases the hammer or striker. DAO simply refers to a subset of double action in which manual cocking of the hammer is not possible so each trigger pull results in cocking and releasing.
With a Glock and a S&W M&P, the striker is not fully cocked until the trigger is fully pressed, hence both these pistols are double action (only). With an XDm, the striker is fully cocked and trigger press simply releases the striker, making it single action.
Glock and S&W Armorer
Glocks: 17L, 22, 22C, 23, 24C, 27, 34