A shower cartridge controls water temperature and flow inside the valve body. Signs of a failing cartridge include difficulty adjusting temperature dripping when off and temperature fluctuations. To replace turn off the water supply remove the handle trim plate and retaining clip. Pull the old cartridge straight out and insert the new one matching orientation. Cartridges are brand specific so note your faucet manufacturer and model. Most replacements cost 15 to 40 dollars and take 30 minutes.