A double-action firearm is one which is capable of cocking and discharging the firearm with a single pull of the trigger.
The majority of handguns are capable of being fired double-action. As far as I know, there are no double-action long guns.
Most guns which can be fired double-action can also be fired single-action. That is, most double-action guns can be cocked in advance of pulling the trigger.
Since cocking the firearm involves loading the hammer or striker spring, a double-action trigger pull involves more effort and force than a single-action trigger pull. The increased effort and force has the effect of making accurate shooting more difficult.