A multisig wallet is a wallet that requires several keys for a transfer. Instead of a single signature, a transaction needs the consent of several keys, and you set the number in advance. The usual notation for this is m of n: in a 2-of-3 wallet three keys exist, any two of them are enough to pay, and a single stolen key does no damage.
That sounds like a pure security question, but it is above all a question of organisation. Multisig shifts the risk away from the fear of losing a key and towards the duty of managing a configuration. Anyone who does not know this sets up a multisig wallet and then loses their Bitcoin to a misplaced file rather than to a hacker. This article shows you when the effort pays off, what you have to back up besides the keys, and at which point most setups go wrong.
What a multisig wallet is and what 2 of 3 means exactly
An ordinary Bitcoin transfer is a single-key transaction: whoever holds the private key to an address can move the balance. The Bitcoin protocol does, however, allow conditions that require several signatures, and such spending conditions are called m-of-n<...
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