A single-signature wallet has exactly one key. If that key is lost, destroyed or stolen, the Bitcoin is gone - there is no reset, no support line and no probate remedy. Multisig removes that single point of failure by splitting signing authority across several independent keys held in separate places.
The most common structures are 2-of-3 and 3-of-5. In a 2-of-3 wallet, any two keys can move funds. That means one key can be lost or compromised without loss of the Bitcoin, and it means a spending policy can be designed around people and places rather than one device in one drawer.
For estate planning, multisig is what makes an orderly handover possible. Keys can be geographically separated, and a documented recovery path can let an executor reconstruct signing authority under defined conditions - without anyone, including BlockByte, ever holding enough keys to spend on their own.