This is a unique identifier name for the Progressbar so you can refer to the Progressbar later in your project.
1. Add a ProgressBar in your main.xml file.
2. In the main.xml file, add the code below between the ProgressBar tags.This will name the ProgressBar myProgressBar.
<Progressbar
...
android:id = "@+id/myProgressBar"
/>
3. The "+" character is required if you have not yet defined that id, if perhaps the element you reference is lower in the .xml file.
Otherwise you if it's already defined above (or included) then you can omit the "+" char:
4. Compile and run!
Next Recommended Article: How to Declare a ProgressBar
Resources:
http://developer.android.com/reference/android/view/View.html#attr_android:id
http://developer.android.com/reference/android/R.attr.html#id
