what is the android setting?

If you turn on this setting,a small white box will appear on your screen and if you click the box the box shows options like home button, back button and screen shot button.I forgot how to turn it on.

my device -samsung galaxy tab s 10.5 android 5.0.2

Go to Settings and search for "Assistant Menu"... should be under Dexterity and Interaction (on Kitkat... not sure about LP or MM).