android.R.id.text1 is just an identifier defined in the Android framework.
In the framework, android.R.id.text1 is an used for TextView views. You can find it in many layouts from the framework (select_dialog_item, select_dialog_singlechoice, simple_dropdown_item_1line, etc.). In Android framework xml, it is represented by @+id/text1.
Hence, if you use one of these layouts and want to change the text, you will need to use this id.
// probably in a custom ListAdapter that uses
View view = inflater.inflate(android.R.layout.simple_list_item_1, parent, false);
TextView textView = (textView) view.findViewById(android.R.id.text1);
textView.setText("Oh no! not hello world again");
Also, you can use this same identifier to identify a TextView (or anything) in your custom layouts. See in the sample “Notepad”, the layout file noteslist_item.xml.
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@android:id/text1"
android:layout_width="fill_parent"
android:layout_height="?android:attr/listPreferredItemHeight"
android:textAppearance="?android:attr/textAppearanceLarge"
android:gravity="center_vertical"
android:paddingLeft="5dip"
android:singleLine="true"
/>
And actually, you could use R.id.text1 as an identifier of anything else, but that would be confusing.