That is the cam that pulls the sear down, releasing the striker, when you pull the trigger to the rear.
It should fit on the side of the pistol, near the top edge of the frame.
You'll have to compress the sear against its spring to get the cam back into place.
Watch this video; the cam is removed at about 1:15, and is reinstalled at about 2:05.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)