What is a screen overlay on a tablet or phone and why do I need to turn it off or on

[Android Central Question]

My tablet keeps saying I need to turn my overlay off what is a screen overlay on a table or phone

 

[ManiacJoe]

A screen overlay is an app that has the power to write over the top of other apps.
Because they can do this, they are a potential security problem, from the perspective of the operating system.
Common overlays are:
- screen dimmers (Lux, Twilight)
- sliding menus, sliding trays
- floating chat bubbles/heads