What does DRL (differential reinforcement of low rate schedule) mean?

I'm readying a study about cocaine administration in rats and it says the study used an ess method called DRL 45s (differential reinforcement of low rate schedule). It says that it results in low rates of responding, as only those responses that occur after a minimum time interval (45s) following a previous response are reinforced. I didn't understood this part of the article. Could someone help me?