There are two security pins that keep the stroller from folding while in use. I am missing one of those pins. It has a molded plastic end on it so that when it is installed properly, it locks in place so that it will not vibrate out. It also has a cord attached so that it will not get lost when it is not installed.

Baby Gear - Cosco - Tandem Stroller 01-075-AFD Twin Seat