MetaMask is a multi-faceted wallet that offers everything from an Ethereum wallet, like handling cryptocurrencies, ERC-20 tokens and ERC-721 collectibles.
It allows users to access Ethereum-enabled distributed applications, or "Dapps" in their normal Chrome browser.
Lets the user create and manage their own identities, so when a Dapp wants to perform a transaction and write to the blockchain, the user gets a secure interface to review the transaction, before approving or rejecting it.