When you release the drag, whatever is in the rectangle will be captured as a screenshot on your desktop. Now you can take that crosshair and click+drag a square on your screen.
That will turn your mouse cursor into a crosshair. MacOS has a very versatile screenshot function baked right into the OS (it's even more powerful if you're running macOS Mojave or later). Note: This section will walk you through capturing a portion of your screen but, if it is easier for you, you can use Shift + ⌘ + 3 to capture the entire screen.
