The bolt holds the round in place ready for you to pull the trigger, which releases the hammer. The hammer strikes the back of the firing pin pushing it through the bolt to strike the primer on the round thereby firing it. The bolt is subjected to the highest amount of stress in your AR.
For that reason, don’t go cheap when choosing your bolt. Rather than go with Carpenter 158 steel for your bolt, FSD recommends 9310 steel. The Carpenter steel is just too brittle. Sure, the 158 is cheaper, but it WILL break – and when it does, your gun is down until you find a new one.