A pop rivet (formally a blind rivet) is a hollow tubular rivet with a pull-mandrel inside. The rivet gun pulls the mandrel through the rivet, expanding the back end and clamping the two parts together. The mandrel snaps off when the rivet is fully set.
Pop rivets are common where access is one-sided (you can't get to the back of the joint), where the parts are too thin to bolt cleanly, or where speed matters more than ultimate joint strength. Common sizes are 3.2 mm (1/8 inch), 4.0 mm (5/32), 4.8 mm (3/16), and 6.4 mm (1/4) diameter.
For aluminium gate work, aluminium pop rivets fix battens or slats to internal cross-rails without needing access from both sides. Stainless rivets are used in marine exposure to avoid bimetallic corrosion.