Think of it as rate of change of one thing w.r.t another.
So, if I get paid P=$2 per foot of ditch, and I dig S=3 feet per day, ##dP/dt = 2, dS/dP = 3##, and ##dP/dt = 6## (i.e., $6 per day). The last one, by the way, is the chain rule.
So, loosely speaking, ##dP/dP = 1## - i.e., I get one...