These mag pouches are designed to hold 2x 15 or 18 dart Talon magazines.
The top flaps are secured with velcro and the pouches are secured to your rig using MOLLE straps. If you're clever, you could even secure them to a belt using the straps.
Did you know that MOLLE stands for MOdular, Lightweight Load-carrying Equipment? I didn't until I just looked it up. The military and their acronyms eh?