MetaMask extension for accessing Ethereum enabled distributed applications, or "Dapps" in your browser! The extension injects the Ethereum web3 API into every website's javascript context, so that dapps can read from the blockchain.MetaMask is indeed a "hot" wallet, in that you use it on a computer connected to the internet. This is usually considered less secure, because if a machine is on the internet, it could be accessed remotely, and keys could be stolen.