What is Firefox Multi-Account Containers used for?

When you sign in to a website, it tells your browser a little secret so that you can continue using the site without signing in for each page you visit. There's normally only one place to put this little secret, so you'd have to sign out of one account before you can sign in to another account. Multi-Account Containers sets up a place to store multiple of these little secrets, so you can sign in to multiple accounts. You'll sign into one account and have your computer put that little secret into one "container," then switch to another container and sign into another account. You can then have tabs using different containers that have you signed into different accounts at the same time.