MetaMask is a plugin that bridges multiple dapps and your Chrome browser without compromising your security, using multiple accounts and without using a full Ethereum node. Users from other browsers expect news soon, while the version proposed by Mozilla still needs some work.
Let's talk about DApps first. The acronyms are for "decentralized applications" or decentralized applications. A DApp (read “D app”, as e-mail) are those that run on blockchain meeting the requirements of being decentralized, horizontal, secure and open. Since the birth of Bitcoin we have seen a large number of DApps emerge satisfying any imaginable need.
MetaMask, developed by cryptographer Aaron Davis and Dan Fiblay, tries to bring blockchains even closer to the user through a kind of DApps interpreter that works as a browser extension, allows access to services, signing blockchain transactions, managing tokens, managing various wallets, all in a friendly and secure interface. The idea is to integrate all the services, including those of Ethereum, in the browser.