What's it for?
Will the shower work without one?
A shower faucet cartridge is a type of valve. It's the part that a single handle faucet is moving inside the plumbing fixture to adjust the flow and temperature. They tend to have o-rings and other parts that fail over time, which can result in a variety of issues, including leaks, and the hot and cold water mixing when they shouldn't, even when the fixture is turned off.