Android - What is a secondary progress in ProgressBar?

Secondary progress vs Progress can be thought of like the downloaded part of a video you're watching vs the complete length of the progress bar

From the source:

There is also a secondary progress displayable on a progress bar which is useful for displaying intermediate progress, such as the buffer level during a streaming playback progress bar.

One can find a fully worked example of displaying such a bar, including screenshots of the appearance of such a bar in a particular style (circular bar which fills from the bottom):

In the image above, the secondary bar is the green area, when it reaches the top, some subtask is complete.