An auto sear ("automatic sear") is a part of an automatic firearm that holds the hammer in the cocked position while the bolt of the weapon is cycling and releases the hammer/striker.
It is basically an internal trigger actuated by the bolt/bolt carrier when placed in-battery.
An auto sear is required in nearly every automatic rifle.
An auto sear allows a semi-automatic gun to be converted into one capable of automatic firing with a single, continuous pull of the trigger.
The Bureau of Alcohol, Tobacco, Firearms and Explosives designated any firearm capable of firing more than one round with a single operation of the trigger mechanism as a "machine gun", and the possession of such devices is illegal under United States federal law if the person in possession does not have the required licensing.