Multisig or multisignature refer to requiring multiple keys to authorize a transaction rather than one key or single signature. Multisig is generally used to evade single-point of disaster, making it considerably more challenging for the wallet to be compromised and divide up responsibility for possession of bitcoins among many persons.
In case of Bitcoin standard transactions, it requires only one signature from the possessor of the private key which is called single signature transaction. However in case of much complicated Bitcoin transactions, it requires signature from multiple persons formerly funds can be transferred, such transactions are called M-of-N transactions.
In case of single private key or signature, it is easier for hackers and attackers to successfully steal your funds while in case of multiple keys or signature, it is more difficult for malware and hackers to steal your funds. The multisignature wallet can be M-of-N type where any M of private keys out of a likely N are mandatory to transfer funds. For illustration 2 of 3 Multisignature wallet have private keys spread across laptop, desktop and smartphone, any of two are required to transfer funds.
Loading multiple keys to M-of-N wallet on diverse positions serves as keys backup. In such case, damage of one key may not result in loss of wallet, since other saved keys can be used to recover the funds from wallet. Multisig has been used for hundreds of years to guard the security of sepulchers holding the supreme valuable relics of saints.