MetaMask is a browser extension that allows you to connect to the Ethereum blockchain and interact with many dapps on the Ethereum network.
MetaMask has the standard functionality of any Ethereum wallet, but also, thanks to customization, it allows you to interact with any erc20 token.
In addition to the main Ethereum network, the application allows you to connect to test networks Ropsten Network, Kovan Network, Rinkeby Network. You can also choose a custom network by configuring it yourself.
In Metamask, you can create a large number of ETH addresses. Since this is an HD wallet, you only need to save one seed phrase to have access to all addresses created in this account.
At the moment Metamask is the most popular application, having installed it, you can always be able to interact on the Ethereum network