A helium release valve is a valve that is designed for watches that are suitable for deep-sea diving. The noble gas helium consists of one of the smallest molecules and is able to penetrate into the watch when diving at great depths (and thus high pressure).
Once in the watch, this gas can’t escape quickly enough, unless the watch is equipped with a helium release valve to let the helium escape from the watch during lowering the pressure. This is necessary to prevent the watch from exploding during decompression. There are helium release valves that you can open and close yourself, but there are also brands / models where the valve automatically opens and closes. Not every watch that is designed for scuba diving is equipped with a helium release valve. In that case, these watches are designed so that no helium can penetrate into the watch.