For security purposes, there can only be one active web session per account / user. If you are logged into your account via a computer and another computer is used to log into your account at the same time, you will be logged out automatically and the newer session will be allowed to continue.
The person with the older / first session will see this message:
The person with the newer / second session will see this message:
You can always reach out to us via email at firstname.lastname@example.org if you feel there's been a mistake or if you're unaware of another active session taking place.
Note: this limit is only applicable to web sessions. This means that if you're also logged into your account via the mobile app, that does not count as another active session - you can be logged into the mobile app via your phone and the website via your computer at the same time.