What is Multisig?
Also called: multi-signature wallet
A multisig wallet requires several independent approvals before a transaction executes, so no single compromised key can move funds on its own.
In practice
A treasury or contract upgrade guarded by a three-of-five multisig is meaningfully safer than one guarded by a single key, but only if the signers are genuinely independent people on separate devices. When checking a protocol, ask who the signers are and what they can unilaterally change.