That is your anti siphon valve, for when you drain the faucet before it freezes.
IDK what brand your faucet is, but here is a repair kit for a similar one: https://www.homedepot.com/p/Woodford-Model-17-3-Piece-Vacuum-Breaker-Float-Kit-RK-17VB/204792497
IDK what brand your faucet is, but here is a repair kit for a similar one: https://www.homedepot.com/p/Woodford-Model-17-3-Piece-Vacuum-Breaker-Float-Kit-RK-17VB/204792497