A check valve is a one-way valve which only allows a fluid or gas to flow one way through a system. Most are spring-ball or piston type which have a compressed piston or ball in them which is pushed up when air flows through one way, and closes again when pressure has equalized or air tries to flow the other way.
Check valves are useful for filling launchers via a male quick disconnect. This way, a ball valve is not needed as a shut off. They can also be used for shutting off a high-pressure hybrid cannon's chamber and barrel from a lower pressure-rated system, (such as a fuel meter, air intake, or Generation II tank).