What is hump under roof shingles?

I believe the hump you are referring to is the rafters of the roof. When they plywood covering the roof framework starts to sag from age, weather or due to poor construction, it will form high and low areas. Roofs that have this type of issue will normally have these humps at regular intervals. If you look in the attic you will see the roofs frame lines up with the spacing between the humps.