What is meant by line drawing?

A red-line drawings is essentially an intermediate drawing that shows corrections or changes to a previous drawing. The term red line literally comes from the (typically) red pen used to amend the drawings by hand. These changes mark-ups show changes made to the drawing subject matter during the manufactuer or construction of the product.

For example, piping drawings were developed at the onset of a project to build a new school. During the course of construction for that school, the piping contractors changes several elements of the design (pipe size, routing, orientation etc). At the end of the project, the constructor would "red line" the drawings showing those changes.

Red-lines are used to then develop record drawings. Record drawings, also called "as-builts" accurately reflect what was constructed and are used for future work and/or reference.