How to use Single Row Marquee Tool
Single row marquee tool can also be used to make a selection. First, select the tool then click in your document then you can see a horizontal line of selection then right click and select Transform Selection. You can see now an anchor point in the center of the line. Now you can extend the selection.
The second use of this tool same thing you can make a line of selection just by click on the image then right clicks and select Free Transform. Now when you extend the selection you can see this tool will make a unique design according to the image part.