>>6093469Panel lines are just little lines molded into the plastic of a kit to add detail, such as where you might find hatches or panels on a mech or the vents on a car. Lining them in ink or paint helps them to stand out a bit and improves the overall look of a model.
Here, look at this picture I stole off the internet. The model on the left has no panel lining done while the kit on the right has some panel lining done. It's a small touch, but it makes a difference in how a finished kit will look.
