Cleco fastners are used to keep holes in two or more pieces of sheet metal aligned while drilling or riviting other holes. They are spring loaded require a pair of cleco pliers to compress them to put them in the hole. When released they will fill the hole forcing both pieces of metal to be stay aligned. There are different sizes according to the hole size and thickness of the total metal stack. There is also the clamp version to clamp sheet metal together with no holes.
If you look at the picture, you see that there are two side tangs with a step and a center tongue. When you compress the Cleco, the side tangs extend out past the center tongue, reducing their width. You would then insert it through the hole and release. The side tangs will then retract, the center tang forces them out against the metal and the step on the tangs will catch the lower sheet and pull them tightly together.