Multisig is short for multisignature addresses. Multisignature addresses are like a joint bank account where two or more private keys are necessary to sign a transaction and spend the funds. A normal bitcoin address has just one private key and if you lose your private key or someone gains access to it, your bitcoins are gone forever. With multisig addresses you have a bitcoin address with 3 or 5 private keys and any two or three of the private keys are needed to spend the coins. Two of three multisig addresses are the most popular where any of the two private keys are required to sign off the transactions.