What is multisig?
Multisig is short for multi-signature. An ordinary wallet has one key: hold it, and you can spend. A multisig wallet has several keys and a rule about how many must sign before a transaction is valid. The usual shape is two-of-three: three keys exist, and any two of them together can authorise a spend. One key alone cannot.
Why would you use it?
A single-key wallet has a single point of failure. Lose that key and the funds are gone. Let someone steal it and the funds are theirs. Multisig removes that knife-edge. In a two-of-three setup, you can lose one key and still recover with the other two, and a thief who grabs one key still cannot move anything, because one signature is not enough.
That safety is not free. You now have several keys to generate, store in different places, and back up, and every spend means coordinating more than one device to sign. The flow is more involved, and the recovery flow especially is something to rehearse while nothing is at stake, not in a panic later. Multisig is worth the overhead once the amount being protected justifies it; for small day-to-day balances, a single key is usually enough.