The belly is the curved section of the cutting edge that sweeps up toward the tip. More belly means more edge in contact when you draw the knife through material, which is why skinning and hunting blades have a lot of it and utility blades have little.
A straight edge cuts by pressing down. A curved edge cuts by rolling through the material as you draw it, so the contact point moves along the edge and each part of the blade does a share of the work. That is why a curved edge feels like it glides through hide, rope or a tomato while a straight one feels like it stops.
Blades built for skinning, like a trailing point, have the most belly. A sheepsfoot or wharncliffe has essentially none.
Lots of belly is best for slicing, skinning and food prep. Little or no belly is best for controlled push cuts, scoring, and any job where you need the edge flat against a surface, which is why box cutters and marking knives are straight.
A drop point is the common compromise: enough belly to slice properly, enough straight section near the handle to still push cut. It is the default for a reason.