Multisig, short for multisignature, means that instead of your bitcoin address being controlled by a single private key, it is controlled by multiple private keys. Unchained Capital is able to do this natively through the bitcoin protocol's pay to script hash (P2SH) feature, which is an address type used in bitcoin. Unchained Capital's vault product currently uses a 2-of-3 multisig, meaning that two of three keys are needed to move bitcoin out of a vault.
This allows for a much greater level of security over single signature schemes because there is no longer a single point of failure. If one key is lost, destroyed, or stolen, it can always be replaced by a new key because Unchained Capital acts as your backup in cases such as this.
You can read more about why multisignature is more secure here: https://unchained-capital.com/blog/why-multisig/