Hey Richard,

A “force stop” is like rebooting an app.  You know how a reboot on your computer, or your phone, usually fixes anything that’s going wrong? That means basically turning it off and turning it on again. Well, rebooting an app can do the same thing.

If an app is misbehaving, you can force quit (stop) it, then start it again. You won’t find a “quit” command anywhere on a smartphone app. That doesn’t mean it’s actually running and doing anything, but it is holding your place. You can use the multitasking screen on either iOS or Android and stop the app, then start it again and that forces it to start at the beginning.

For example, if I’m looking at Google Photos and I’m not seeing some pictures I know are there – I want to tell the app to refresh, go look for those new pictures. But, there is no refresh command, so I can force quit, then restart.j

On Android
  – tap the “recent apps” button. It’s beside the Home button at the bottom of your phone’s screen. 
  – Tap the X on the misbehaving app, or swipe it to the side – off the screen

On Apple iOS
  – double tap the home button to see all recent apps
  – swipe the misbehaving app up off the screen

Here’s  an article from Apple help on force-stopping apps.