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 those applications are performed in this thread.
For instance, lets say your application is a single Activity class. Then all of the lifecycle 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.
For more info on your applications Processes and Threads click here.
When you explicitly spawn a new thread to do work in the background, this code is not is not run on the UIThread. So what happens if the this background thread needs to do something that changes the UI? This is what the
runOnUiThread is for. Actually you're supposed to use a Handler (see the link below for more info on this); it provides these background threads the ability to execute code that can modify the UI. They do this by putting the UI-modifying code in a Runnable object and passing it to the RunOnUiThread method.
For more info on spawning worker threads and updating the UI from them click here
I personally only use the RunOnUiThread method in my Instrumentation Tests. Since the test code does not execute in the UIThread, you need to use this method to run code that modifies the UI. So, I use it to inject click and key events into my application. I can then check the state of the application to make sure the correct things happened
For more info on testing and running code on the UIThread click here