You install a bipod on the handguard of your replica and ensure that when you place your replica on the ground it is stable. The bipod makes it easier to aim when you are on the ground and makes each shot more stable. The bipod is mainly used for the sniper because with a bipod you have to make as little effort as possible in order to be able to aim well.
If you would like a bipod on your replica but do not have rails, you can install the rails mount on your replica. QTAC has a wide range of mounts for your replica.