A multi-signature address is an address that is associated with more than one ECDSA (Elliptic Curve Digital Signature Algorithm) private key. The simplest type is an m-of-n address - it is associated with n private keys, and sending coins from this address requires signatures from at least m keys. A multi-signature transaction is one that sends funds from a multi-signature address.

The primary use case is to greatly increase the difficulty of stealing the coins. With a 2-of-2 address, you can keep the two keys on separate machines, and then theft will require compromising both, which is very difficult - especially if the machines are as different as possible (e.g., one pc and one dedicated device, or two hosted machines with a different host and OS).

It can also be used for redundancy to protect against loss - with a 2-of-3 address, not only does theft require obtaining 2 different keys, but you can still use the coins if you forget any single key. This allows for more flexible options than just backups.

It can also be used for more advanced scenarios such as an address shared by multiple people, where a majority vote is required to use the funds.

As of now, MC is fully capable of creating 2 of 2 signatures as long as the third signature is not set. MC is also capable of creating 2 of 3 signatures which you can verify even with a 0 balance.

As a result, MultiCoin is currently the most advanced, most secure coin the world has ever seen.