What does the animated bar in Visual Studio Test Explorer indicate?

In Visual Studio's Test Explorer window, there is a an animated bar that moves left to right. What does this indicate? Intuitively it feels like it should be indicating that tests are running, but that's not the case:

Probably the most significant thing it represents is the "Test Discovery", but also when the build starts on a test project that hasn't been built yet I believe it also begins to animate. Once the tests start to actually execute, it becomes a "standard" left-to-right progress bar.

Here is a .gif of the bar going left to right with the information label changing indicating what it is doing:

I can't tell exactly, but in your screenshot, you seem to be missing this information label (maybe your Test Explorer window is too small?)