MetaMask login serves as an Ethereum wallet: By installing it, you will get access to a unique Ethereum public address, with which you can start sending and receiving ether or tokens.But MetaMask does something more than an Ethereum wallet. As a browser extension, it can interact with the current webpage you’re browsing. It does so by injecting a JavaScript library called web3js in every webpage you visit. Once injected, a web3 object will be available in the JavaScript code of this website. To have a look at what this object looks like, just type windowweb3 in the Chrome or Firefox DevTools console, if you have MetaMask installed.