These are the lines that form between the sides of the nose and corners of the mouth. As we age, the fat present in the cheek drops due to gravity and structural changes occur causing the lines to become more obvious. You may notice that makeup collects here when it never used to.
Part of what gives these lines or folds their prominence is the fact that, as the cheek tissue drops, a shadow is created in the fold making it look more exaggerated. Dermal filler can be injected into the region of the fold to flatten the area a little, reducing the depth of the fold and the shadowing. The end result is a smoother and more youthful appearance.