[oscarhong]

It seems like an indicator of the current progress of the stage.

[antrix]

I thought of it as a glitch too.. till I made enough progress in the game to find that it was a progress bar. Would be better if it started from the right of the 'up' button and not from the very edge of the screen.

[grantland]

Yeah we noticed some people didn't get it right away. We tried a few designs and this one was the best D:

[Off]

Indeed, it's a progress bar.