Multisig wallets are wallets that require a defined amount of different signature to execute a transaction.

The most used multisig on Ethereum is Gnosis-safe multisig. It’s a smart contract wallet.

At the creation you define the other addresses/wallets that will be co-owner of the multisig with you as well as the signature treshhold to execute a transaction.

For example if you are 5 signers you can define that transactions will be executed if 3 wallets on 5 have signed.

Multisigs allows you to share treasury security and smart contract ownership between multiple user/team members reducing trust assumptions and security vulnerabilities.