The term multisig stands for multi-signature, which is a specific type of digital signatures that can be created through the combination of multiple unique signatures. The multisig technology makes it possible for two or more users to collectively sign digital documents or cryptocurrency transactions.
In short, the funds stored on a multi-signature address can only be accessed when 2 or more signatures are provided at the same time.
As a simple analogy, we can imagine a secure deposit box that has two locks and two keys. One key is held by Alice and the other one is held by Bob. The only way they can open the deposit box is by providing their both keys at the same time, so one cannot open the box without the approval of the other.
Since multisig addresses require more than one signature to transfer funds, they are also suitable for businesses and corporations that may want to store funds on a shared wallet. This would mitigate the risks that arise by keeping the funds in the hands of a single person or by handling a single private key to multiple individuals at the same time.