Line-weights is the term that the artists use to refer to the thickness or the strength of the lines in the drawings. Firstly, the artist draws several lines with different intensity and different thickness. He does that to simplify the explanations about the lines which have different role in the drawings. For example, the edges that are in the front are light and less stressed. The corners which overlap with the other parts are the just a little bit more intense. The thicker lines are representing the back and the bottom of the figure.
Line-weights Instructions by Spencer Nugent
See the tutorial below and get the idea about the type of the lines and where to use them.