The easiest way to put this, is think how it works on Mobile today. Same concept. All accounts you add and tenants are very quick to switch. As well as you can take meetings and calls etc. from other tenants without leaving the tenant you are in. You get notifications for all logged in accounts/tenants and can open those threads. It's a massive improvement over multi-tenant in the 1.0 client.

Hi, With new Teams, you can be signed into multiple organizations and accounts simultaneously and receive notifications for all your accounts, no matter which one is currently in use. New Teams will enable you to collaborate more effectively across organizational boundaries by being actively signed into multiple accounts at the same time and receiving real-time notifications no matter which one is currently in use. You can seamlessly engage with users across multiple accounts and organizations without having to drop out of a call or meeting, ensuring no disruption to your workflow. Check out the video: https://www.microsoft.com/en-us/videoplayer/embed/RW10Dhz