A multisignature wallet, often called a multisig wallet, is a specific kind of wallet requiring more than one signature for executing Bitcoin transactions. Common multisig configurations are either 2-of-3 or 3-of-5 signatures and are sometimes linked with companies that offer custodial services for a wider range of services.
Multisig services vary, with options ranging from free and simple payment intent verification via 2FA (an example being Blockstream Green) to more complex, paid services offered through companies like Unchained Capital, designed to assist in managing your bitcoin throughout your lifetime.
Most companies specializing in custody (like Unchained Capital) typically hold fewer keys than the threshold number required to access the funds, ensuring they cannot misappropriate your money. Similarly, you would also store fewer than the necessary number of keys online, safeguarding your funds from being stolen if someone manages to gain control of your wallet, whether through theft or physical coercion.
As long as you maintain control over the wallet, you can work in conjunction with the custodian on the basis of mutual trust whenever you wish to make a payment, together achieving the necessary number of signatures. Conversely, a thief lacking this trust relationship would be unable to access the funds. You would also possess an additional key stored offline, enabling you to access the funds even if the custodian is unable or unwilling to cooperate. Furthermore, in the event of the owner's death, the custodian, legal representatives, and notaries can collaborate to ensure the bitcoin is appropriately distributed to the heirs as intended.
However, using multisig wallets is certainly not exclusive to custodial services. Many utilize a multisig setup for the cold storage of their bitcoin to further safeguard against the loss or theft of one of their keys. Employing different hardware wallets in your multisig setup also offers protection against malicious software that might covertly attempt to extract your keys or subtly alter your transactions.
TL;DR: A multisignature (multisig) wallet requires more than one signature for Bitcoin transactions, enhancing security. Common multisig configurations in wallets are 2-of-3 or 3-of-5 signatures. Many Bitcoin companies will offer multisig services ranging from free payment verification to paid lifetime Bitcoin management. The latter, which are custodial services, often hold fewer keys than needed to access funds, preventing misuse. Additionally, multisig wallets are a popular option in the cold (offline) storage of bitcoin, mitigating key loss or malware risks.