A Multisig is a digital encryption scheme that is used to create applications for multi-party wallets and other abstractions that require authorization from n-of-m participants.
Susan is a wheat farmer. Barry is a food grocer. Barry asks Susan to grow & pick him 100 pounds of wheat and in return, he will pay her 10 Bitcoins. Susan would love to take his offer but is worried that is she does all this work that in the end barry might not hold up his end of the Bargain, heck he might even just spend all his money by the time she grows everything. So, Susan and Barry invite Artemis into the situation. (We assure Artemis is neutral and does not care about the outcome, he simply must observe & report what happens.) The 3 of them create a multi-signature wallet that requires 2-of-3 signatures to unlock. Barry deposits the 10 Bitcoins to the multi-sig address and now they wait. Time goes by, Susan grows and collects the 100 pounds of wheat and goes to Barry. She gives Barry the products as promised butt Barry decides to kick her out! While that is not ok, Artemis saw that Susan Delivered. The two of them go to the Multi-sig account and claim the 10 BTC. Barry, being only 1-of-3, cannot unlock the wallet.