Mobile devices are getting pretty fast, but they aren’t infinitely fast yet. If you want your app to be able to do any serious work without affecting the user experience by locking up the interface, you’ll have to resort to running things in parallel. On Android, this is done with “threads”.
Grab yourself a cup of coffee and read this post line by line. I’ll introduce you to the concept of threads, talk about how Java uses threads and explain how “Handlers” in Android help with threading.