Welcome to the forum!
That's the page border. It's not actually part of whatever drawing you're going to make. It's just there as a guide. In certain situations, where you might be using your finished drawing, it might not show up if it's not inside the border.
You can hide it, if you don't want to see it or you don't have use for it. Go to Document Properties > Page tab and way down near the bottom, there's a checkbox that you need to uncheck.
Currently there's a small bug where you might not be able to see that option, if your screen is small (a regular laptop screen is apparently considered small these days). It will be a long explanation, so I'm not going to type it all out unless you need it. So if you can't see that option, let us know, and we'll tell you what to do.
