A multisig is configured as m-of-n: two of three, three of five, and so on. Funds move only when enough distinct keys sign, so losing one key is recoverable and stealing one key is useless.
Treasuries, funds and DAOs use multisig as standard practice, and individuals use it for large holdings. The cost is friction: every transaction needs coordination, and a badly designed setup can lock funds if enough signers become unreachable.
Example: a 2-of-3 with keys at home, in a safe deposit box, and with a trusted party survives a house fire and a single theft. A 3-of-3 survives neither, because any lost key is fatal. Model the failure cases before choosing the numbers.
Related: smart-contract-wallet, self-custody