Multisigs are smart contract wallets that custody assets. They have embedded logic that broadcasts transactions once they have received sufficient signatures. For example, a multisig may need 2-of-3 keys to sign a transaction before broadcasting it to the blockchain.
For those familiar with traditional corporate governance, this is similar to requiring multiple signatories to approve a company resolution before anything important can be executed. Multisigs have become a critical piece of infrastructure, facilitating trustless collaboration for asset management for multi-billion dollar DeFi protocols, bridges, investment DAOs, funds, and even individuals.