What are overlays on mobile?
A screen overlay is a part of an app that displays over other apps, such as the chat heads in Facebook Messenger.
What is an overlay in Android?
As a key UI feature of Android, overlay enables one app to draw over other apps by creating an extra View layer on top of the host View. While greatly facilitating user interactions with multiple apps at the same time, it is often exploited by malicious apps (malware) to attack users.