The primary bevel is the main grind that takes the blade from its full stock thickness down toward the edge. Its shape, flat, hollow, sabre or convex, is what people mean when they name a knife’s grind.
Everything behind the edge has to pass through the material you are cutting. A primary bevel that leaves the blade thin behind the apex glides through; one that leaves it thick wedges the material apart and binds. This is why two knives with identically sharp edges can feel completely different.
See flat, hollow, sabre and convex for the four you will meet.
Ordinary sharpening only works the secondary bevel. Over many years the secondary bevel creeps wider as the blade wears back, and eventually the knife starts cutting worse even when it is sharp.
At that point the fix is to thin the primary bevel, which means removing a lot of steel. It is a job for a belt grinder or a professional, not a pocket stone.