This notification happens when the OS tells us that the phone is running low on memory.
If you have a lot of apps open on your phone or there's a badly behaving app, you'll eventually get this message until you close the apps.
In order to close the apps, prior to iPhone X, you have to press the Home button twice, then you can swipe away apps to close them. Starting with iPhone X with no Home button, you have to swipe from the bottom of the screen to get the list of apps, then you can swipe them away to close them. Alternatively, you can restart your phone.