What is the 12-word secret phrase?

Can you please explain what exactly is meant by the term "12-word secret phrase"? I've heard it mentioned in the context of cryptocurrency and digital wallets, but I'm not entirely clear on its significance. Is it a unique set of words that serves as a security measure, or does it have some other function? I'd appreciate it if you could elaborate and provide any additional context that might help me better understand this concept.