What's the meaning of android:progressBarStyle attribute in ProgressBar?

The four attributes that you mention can be applied to a ProgressBar's style like so:

style="?android:attr/progressBarStyleHorizontal"

The style constant android:progressBarStyleHorizontal is your typical incremental progress bar:

While the other three are varying sizes of the same circular progress bar:

style="?android:attr/progressBarStyleSmall"

style="?android:attr/progressBarStyle"

style="?android:attr/progressBarStyleLarge"

Update:

According to adamp's comments:

These are attributes of the theme that point at themed styles you can use for progress indicators...They are not attributes for ProgressBar itself.

In case if someone is looking for full block of code

<ProgressBar
android:id="@+id/ProgressBar2"
style="?android:attr/progressBarStyleLarge"
android:layout_width="match_parent"
android:layout_height="wrap_content" />