Whats a bleed down valve. How to use it

Typically a bleed valve is in the High Pressure Side of the system, so that the pressure can be relieved so that the connection to your tank or gun can be removed after being filled or pressurized. When there is high pressure on the line you can't unscrew or disconnect most connections due to the pressure keeps them from being released. In other words the bleed screw or valve can be opened to relieve the pressure from the system. Hope this helps.