It's a debt you have to someone because they let you smoke a bunch of their weed and now you have to pay them back with your own.
Dave and Carlos are hanging out on a warm summer's night. David turns to Carlos and whispers seductively into Carlos' nervous ear, "May I smoke some of your weed, my good ol' friend". Carlos turning away in anger thought to himself, "Fuck I have a dank debt to him. He let me smoke all of his dad's weed". Carlos turns back to Dave and says reluctantly, "I guess you can have some". With a look of shame, Carlos handed him his dank and a tear almost fell out of his eyes. Almost because Mary Jane dried his eyes out.