The secondary bevel is the narrow bevel at the very apex of the blade that forms the cutting edge. It is the surface your sharpening stone actually contacts, and on most factory knives it is well under a millimeter wide.
Grinding the whole primary bevel to a fine apex would leave the edge extremely thin and fragile, and it would make sharpening a huge job every time. A separate narrow bevel at the tip solves both problems: it is quick to renew and it can be set at a more durable angle than the primary geometry. Whether such an angle is quoted for one side or for both together is the question behind inclusive angle.
Some knives, called zero-ground or scandi-ground, skip it and sharpen the primary bevel right to the apex instead.
Every sharpening removes steel and moves the apex back into thicker stock, so the secondary bevel gets a little wider each time. That is normal. When it becomes visibly broad, the knife cuts worse even when sharp, and the primary bevel needs thinning.
Keeping a consistent angle slows this down more than anything else you can do.