I have been thrust into an Adobe XD project and I've been using Sketch forever. There is an icon that I don't understand or know what it does I'm wondering if someone can educate me....

It's a little blue square that looks like a two line hamburger menu and a dotted line spans my artboard horizontally from it - I can move it up and down along my artboard height.

I'm 'guessing' it has something to do with prototyping as that seems to be the hook I'm seeing online - though I can't see this icon anywhere online?

Any thoughts?