Information on this page is relayed from MetaMask’s relevant official support article (linked below).

https://metamask.zendesk.com/hc/en-us/articles/360015489531

MetaMask is a web browser extension and mobile app that allows you to manage your Ethereum private keys. By doing so, it serves as a wallet for Ether and other tokens, and allows you to interact with decentralized applications, or dapps. Unlike some wallets, MetaMask keeps no information on you: not your email address, not your password, and not your Secret Recovery Phrase or other private keys. You retain all power over your crypto-identity.

Steps to install

  1. Visit https://metamask.io/
  2. Click on "Download" in the menu bar
  3. Click "Install MetaMask for <your browser**>". You will be directed to your browser’s web store.
  4. Click “Add to <your browser>”. Or if you’re using Microsoft Edge, click “Get”.
  5. In the pop up that appears, click “Add extension”
  6. After adding the MetaMask Extension, you will be directed to a MetaMask setup page in a browser window. Proceed through setup flow until you are asked to “Import wallet” or “Create a wallet”.

    1. If you are new to MetaMask, follow this guide:

      Set up a wallet in MetaMask

    2. If you have wallets under a MetaMask on a different device, follow this guide:

      Import wallets from a different MetaMask client

After adding MetaMask Extension, MetaMask will automatically open.

Tip: Make the extension easily accessible by pinning it or setting it to “Show in Toolbar”.

Notes

When adding MetaMask to your browser, you may ask why you’d have to approve an extension to “read and change all your data on the websites you visit”. Get ready, the answer is technical:

In order to enable dapps (decentralized applications) to access the blockchain, MetaMask needs to inject a Web3 JavaScript object into each page. By doing so, it will not change the website, but merely allows it to access the network.

If you are still not convinced, a good way to experiment and manage your browser is to sandbox your MetaMask: create a separate browser profile so that MetaMask is only installed there. This will let you get used to MetaMask and Web3 in an environment that's separate from your existing web identity.