A section line on a drawing shows?

I know what a section is but not quite sure what you mean when you say section line- but a section shows you the detailing of any building element cut through the middle. Imagine making a sandwich and cutting it through the middle as you would then all the layers you see, like the bread, lettuce, tomato meat and then the other slice of bread: what you are looking at is the section view of the sandwich. Now do this with any building/ building element and this is what you get. for a better idea, google image search - solid floor section detail or google search the section detail for the element you are looking for.