Android UI thread / main thread

UI thread 

When an application is launched, the system creates a thread called "main" for the application. The main thread, also called the UI thread, is very important because it is in charge of dispatching the events to the appropriate widgets, including drawing events. It is also the thread where your application interacts with running components of the Android UI toolkit.

Android UI principle: single-threaded model for the UI: the Android UI toolkit is not thread-safe and must always be manipulated on the UI thread.