Construction Lines are the first step of sketching. A construction line may be used in a prototype of the sketch to get a good form of the thing being drawn or part of the drawing as the first step of the drawing procedure. There can be one line or many lines. The next step is Detail.
If you are using one line in a sketch, that means you are drawing a 2D object. Some people new to the art of sketching might just use the one line to get used to the are. One line is also used to show that the image has more than 1 side, by meaning a side as a side of a square.
Many lines are used to show that there is thickness in the image being drawn or that the image is wide. If using multiple lines when sketching, you should not press down as hard as normal, but lighly sketch on the paper. When you use many lines in drawing, you are most likely telling the looker that the object is 3D and is round/oval shaped.