A user termination is when the user force quits the app (for example, on iOS it is when the user swipes up to quit the app or closes the app in the task switcher). 

There is typically some percentage of the user base that will terminate the app regardless of whether or not there is an issue - which is why we look at the difference between the rate at which people background the app on a specific page vs terminate on the page. 

If there is a big difference in the two rates, it often indicates that the page is frozen or blank.  This is indicated by high correlation values (marked in red) on the User Termination page.