An inexpensive (Barnett Sportflight) archery set I bought includes this little piece, about 1.5 inches long. Can anybody tell me what it is for?
Without seeing all the parts for context, I'd say it looks like a budget sight. The trapezium cutout will attach somewhere on the front of the riser and slide up and down (to adjust vertical aim), and the screw part will move the sight left and right (to adjust horizontal aim).
To sight, line up the ball over the centre of the target. Shoot some arrows. If they're higher than centre, move the sight up; if they're low, move sight down. If they're right of centre, move sight right; if they're left, move sight left. If they're all over, keep practicing until you can consistently group 3 arrows together :-)