Metamask is a digital wallet for receiving, sending and storing cryptocurrencies issued on Ethereum Blockchain.
It is also a browser extension which enables it to connect browsers with Ethereum blockchain. Its means that users can fully interact and browse the Ethereum blockchain without downloading the whole blockchain. In simple words, with Metamask you can browse the Ethereum blockchain from your browsers.
A basic benefit of Metamask is that you can explore Dapps with Metamask without running a full node. Metamask can be used to connect with various decentralized apps developed on Ethereum blockchain and can be used for sending and receiving assets through these Dapps.
Metamask does not store its keys on its own servers. Metamask keys are stored locally and controlled by the users. Even though it is a web based wallet, metamask has not suffered any serious hack.
Metamask is available for three different web browsers, Chrome, Firefox and Brave. It can be used for storing ether and Ethereum token keys on these three browsers.