The stopcocks are the common faucets often made of bronze that are operated by a screw control that closes the pipe by a seal.

It is a good rule to operate them from time to time to prevent the command from locking. If you happen to sprinkle the command with bleaching oil (like svitol) and try to restore its functionality.

If it is necessary to replace the seal (the faucet leaks), remember to intervene before closing the main faucet of the system to block the water flow upstream.