I would like to know what function did the ribbed portions on Saturn V exterior serve, such as the the intertank section and forward skirt on Stage 1. Thank you!
That's called corrugated sheet metal. The function of the corrugations is to make the material stiffer in the direction of the corrugations.
Although instead of corrugations, stringers were often used to serve the same purpose, such as on the Saturn V:
This solution is easier to fabricate than pressing corrugations into a conical sheet of metal.
They were used on those sections where the skin isn't pressurized: the tanks are stiff enough not to need stringers, the non-tank sections (interstage, intertank sections) did need them.