If you have been inactive on the page for a specific period of time, your session will expire, and you will be disconnected from your account. This happens for security reasons in order to avoid a third party accessing a logged-in account.
You will then need to log in once again to be able to use the site and bet. You will also be disconnected from your account if you sign in using a different device, as you can only be logged in to one device/browser at a time.