When you start a study session, the app will periodically "check in" to the backend server indicating that you are still in a study circle. In order for this to work, the app needs to run in the background.
You do not need to keep the app in the foreground (on your screen) while you're in a study session, but you do need to have it running in the background. This means you're able to use other apps while you have a study session active.
Force closing the app from app switcher (swiping it away) will end your active study session. It is not recommended you do this as data could be lost. Always end study sessions by using the end button found on the study map in the application.