Selection shown above. The toolbar fly-out for the marquee tools is shown below. The Crop Tool is at the far right, but does not apply to this subject. Stripes created with the Single Row and Column Marquee tool. You can create the selection even when the cursor is off the canvas.
Masks are selections and the two articles work hand in hand.
When I present basic techniques, I will always start right at the beginning. Photoshop offers several selection tools. The selections shown at the left are created with the four marquee tools located in the top left corner of the toolbox. Rectangle, Elliptical, Single Column and Single Row. To make a rectangular or circular selection, click and drag with the desired marquee shape tool.