activity-based timeout

if I can qualify a little more,
we know that this Activity Based Timeout is for idle-time out on browser.
So if I do not do anything on the app opened on browser for eg., 15 min., then after 15 min., I will see the Azure-AD popup to sign in again.

The doc says that application need to support activity-based timeout.
I am not sure, what kind of support AAD is expecting from the web-app and secondly how does AAD find out that I am idle on the app-browser ?

Thanks.