What is meta mask io?

MetaMask.io, can you tell me more about it? I've heard it's a popular tool in the cryptocurrency world, but I'm not entirely sure what it does or how it works. Is it a wallet, an exchange, or something else entirely? I'm curious to learn more about its features, benefits, and any potential drawbacks or risks associated with using it. Can you provide me with a concise yet informative description of MetaMask.io?