A double action pump is a type of air pump that allows air to be pumped into an inflatable item (such as an inflatable stand up paddleboard or air mattress) with both the upstroke and downstroke of the pump handle.
In a double action pump, there are two valves: one for letting air into the pump and one for letting air out. As the pump handle is pulled up, air is drawn into the pump through the inlet valve. As the pump handle is pushed down, air is forced out of the pump and into the inflatable item through the outlet valve. This means that the pump can inflate the item more quickly than a single action pump, which only inflates the item on the downstroke of the pump handle.
Double action pumps are often used for inflating larger items that require a lot of air, as they can inflate the item more quickly and with less effort than a single action pump. They are also useful for inflating items to a higher pressure, as the extra air being pumped in with each stroke helps to increase the pressure more quickly.