A 3d cardboard box has 6 sides, each of which are rectangles. If you unfold the 3D box, and flatten it out, then you'll be left with 6 rectangles such as what you see in the attachment below. This is one way to unfold the box. This flattened drawing is the net of the 3D rectangular prism. You can think of it as wrapping paper that covers the exterior of the box. There are no gaps or overlapping portions. If you can find the area of each piece of the net, and add up those pieces, that gets you the total area of the net. This is the exactly the surface area of the box.
In the drawing below, I've marked the sides as: top, bottom, left, right, front, back. This way you can see how the 3D box unfolds and how the sides correspond to one another. Other net configurations are possible.