MuSig is a protocol for creating Taproot multisig public keys and signatures. MuSig makes use of Schnorr signature and public key aggregation, and thus will only be possible with the activation of the Taproot upgrade.
MuSig is special in that a resulting multisig transaction is no longer discernible from a single signature transaction. This is because MuSig combines the individual public keys of each party to create a single public key. When bitcoin is spent from this public key, the spenders are not forced to reveal their individual public keys. Instead, they collectively create a single signature valid for the public key they created earlier. This is not the case for typical multisig transactions, which use P2SH scripts and force the signatures and public keys of each signer to be revealed on the blockchain.
MuSig presents a significant privacy improvement over the current multisig implementation, and not just for MuSig users. MuSig will undermine many heuristics currently used for chain analysis by removing any differentiation between single signature and multi signature transactions.