What to do about the cracks on my wood floor?
First, this doesn't look like like solid wood flooring.  At best, this is an engineering wood flooring (plywood).  Mostly likely, since there are 3 boards to a plank, this is a laminate floor.

Second, the finish, in general, is in really bad shape.  Any type of spot refinishing is just going to stick out, maybe as bad as the damage you are trying to cover.

Since this floor is most likely laminate, the typical means of refinishing a floor (sand and refinish) is out the question.  The dark areas is primarily dirt, but using any type of harsh cleaner to get it out and then reseal is also out of the question.  The cleaners will mostly destroy what is left of the finish and the wood look.

If this damage is localized, lets say the entry way, you can always use an area rug to cover.  This I feel is your best option.