Every transaction in a blockchain is signed by the sender. The signing of transactions is done using cryptographic signing algorithms, as described in episode 6 of the podcast.
In some scenarios, to perform an operation on the blockchain, there is a need to have more than one signatures on a transaction. Think of bank accounts with joint holders. Similarly, blockchain wallets could also be owned jointly by a group of accounts. These wallets are called multi-signature wallets or multi-sigs.
Episode 25 - Multi-Signature Wallets (MultiSigs)