What is a multisig wallet?
A wallet whose coins need signatures from several keys, such as 2 of 3, so no single key can spend alone.
An ordinary wallet needs one key's signature. A multisig wallet names several keys and a threshold: a 2-of-3 setup spends with any two of the three. People use it for shared funds, and so that one lost or stolen key means neither lost nor stolen bitcoin.
The spending rules sit in the payment itself. With TaprootTaprootA 2021 upgrade that added Schnorr signatures and addresses starting bc1p, making complex payments cheaper and more private., the keys can combine so that a multisig spend looks like an ordinary single-signature one, which helps privacy.