what is UIthread in android?

The UIThread is the main thread of execution for your application. This is where most of your application code is run. All of your application components(Activities, Services, ContentProviders, BroadcastReceivers) are created in this thread, and any system calls to As for example in a  single Activity class. Then all of the life cycle methods and most of your event handling code is run in this UIThread. These are methods like OnCreate, OnPause, OnDestroy,OnClick, etc. Additionally, this is where all of the updates to the UI are made. Anything that causes the UI to be updated or changed has to happen on the UI thread.