MetaMask is an Ethereum browser extension. It's built on top of the Web3 API, so it enables you to interact with the Ethereum blockchain directly from your browser, like a normal browser. It's a simple way to keep your passwords, private keys, and tokens private, by hiding them in your browser. It also has a nice UI that looks like the one on the blockchain.

This is the development build of the MetaMask extension. It contains some unreleased APIs that are not ready for general use, and is intended for use only by developers. It's built in a way that allows developers to provide feedback, and it will not cause any harm to users.

What is the extension?

It allows you to interact with the Ethereum blockchain directly from your browser. The extension injects the Ethereum web3 API into the javascript context of every website. So it's like having your own web3 API, which you can use to interact with the blockchain, like a normal browser.