Screen overlay is a feature of modern Android smartphones and tablets that allows compatible applications to be displayed on top of other applications. It is commonly seen in applications such as Facebook Messenger, where chat heads can appear when you receive a message, and Twilight, which runs color filters over the screen to reduce eye strain and block out harmful blue light at night.

Screen overlay has its uses, and many advanced features of modern applications would not function without it. It allows them to "draw" over the application you are using.

Screen overlays let you continue using an app regardless of what else you’re doing on your device. Without that ability, you’ll have to manually open the app if you want to receive updates from that app; apps that make visual adjustments to your phone won’t work at all.

However, a hacker can use a screen overlay maliciously. For example, a screen overlay that hides a consent dialog can trick you into agreeing to something you are not aware of. To prevent this, an Android device warns you that it is using a screen overlay and blocks you from going through. You must disable the screen overlay feature to proceed.