Watch-only is a term used to describe crypto wallets that allow you to keep track of your crypto in cold storage while disabling the ability to spend funds. This is because watch-only wallets do not store or use private keys, and are therefore unable to authorize any spends on behalf of the user.
These wallets use only public keys, allowing them to show users their balances and receive bitcoin. A watch-only setup enables use-cases such as being able to monitor wallets while travelling, keeping watch over a friend, or family members’ funds, and is an optimal part of cold storage. However there are multiple watch-only wallet crypto scams one should be cautious off.