Multisignature, commonly referred to as multisig, refers to requiring more than one key to authorize a Bitcoin transaction. It is generally used to divide up responsibility for possession of bitcoins.
A multisignature address is an address that is associated with more than one private key. The simplest type is an m-of-n address - it is associated with n private keys, and sending bitcoins from this address requires signatures from at least m keys. A multisignature transaction is one that sends funds from a multisignature address.
Multisignature DEMO using Trezor and Electrum starts at 2:42:57