Gmail arrow, what is that?

Look at this Arrow in the inbox

Every time I log into Gmail, the first thing I see is this:

And specifically, this thing:

What is it? What is it pointing to? What does it mean? Clicking on it does nothing. Not even a tooltip explaining what it is. I don’t have much to say about it because to this day I don’t know what it is. And by the way, I know is has to do something, my point is that it’s not easy to figure out what.