Certain types of apps like VoIP apps, and music or audio apps in general, may continue running in the background on your iPad or iPhone, even when those apps are currently not visible on your screen. Since especially audio apps can be quite intensive regarding hardware resource consumption, it is essential for you as user to control them, and if required to stop them from running in the background, in order to ensure that you have enough hardware resources left for your current audio app.
List of Background Apps
To see a list with all apps currently active on your system, press the home button of your iPad / iPhone once, so you can see the iOS desktop, then press the home button twice. A horizontal list with app tiles will appear on your screen. Each tile represents the current screen content of the respective active app. You may slide your finger horizontally over the screen to browse through the entire list of active apps. Not all of them are probably continuesly running in the background, many of them are suspended by iOS until the respective app is recalled on screen. However in case you encounter symptoms of a high system load on your device, i.e. misbehavior of some app, crashes, your device keeping hot or draining the battery quickly, then you should explicitly stop those apps, as described next.
Stop Background App
To force quitting an app, no matter if suspended or currently running in the background of your system, scroll horizontally through the list, then slide the respective app's tile upwards until it vanishes off screen. The app is now completely stopped and does no longer reserve or consume any hardware resources. Repeat this procedure to empty the entire list of active apps, in case you encountered an unexpected high system load under certain situations.