A common practice in games is to pause the game when it's been backgrounded, rather than making the user forfeit outright. This behavior can actually be a concern when it comes to fairness. All iOS phones have an application switcher which displays the last thing that was on screen before the app backgrounded. If the game is paused but the user can continue to see critical information in the app switcher, then users will abuse backgrounding the app as an unfair way to gain advantage over other users.
To solve this problem, you need to hide the in-game elements from the screen when the application is backgrounded. You can implement the function applicationDidEnterBackground or listen for the delegate UIApplicationDidEnterBackgroundNotification to detect when your application is entering the background. When that event fires you should set all of your critical in-game UI to hidden.
Note: Make sure that you also implement the applicationWillEnterForeground function or the delegate UIApplicationWillEnterForegroundNotification as well, to un-hide your UI when the application returns.
For more detailed information, please see the official Apple article "Preventing Sensitive Information From Appearing In The Task Switcher"