In Android App Development Default ListView we used new new ArrayAdapter(this, android.R.layout.simple_list_item_1, somearray); ->

There are several important concepts here, but let’s look at the R bit first. R (or, R.java, if you prefer) is a dynamically created class which gives you access to the resources in your project. If interested, you can read more about accessing resources via the R class.

We can use the R class to get a resource ID by specifying a resource type and a resource name. The resource type would be something like string, drawable, or layout – matching the various resource types you see in your project. And thus, the layout part in android.R.layout.simple_list_item_1 simply specifies that you are referring to a layout resource.

More here: http://www.raywenderlich.com/78576/android-tutorial-for-beginners-part-2