This is the USGI standard issue M249 SAW gunner pouch set.
This set of pouches allows a M249 gunner to carry the ammo they need right on their kit. You can attach these pouches to your MOLLE surfaces, and secure them closed with a buckle. Also, the 200 round pouch can fit a M249 ammo drum and secure closed.
You can use these pouches as great general pouches as well. Though designed to hold ammo, the pouches can secure quite a few common outdoor items.