Making a request of an individual (in many cases, your drug dealer) to accept your promise to pay in the future in exchange for immediate access to an item that you would like to purchase (in most cases, you would be looking to purchase drugs), in the event that you do not have immediate access to the funds necessary to cover the cost of the drugs you need &/or want, at that moment.
If I would have thought about it, I wouldn't have made that deal, as he doesn't have a way to repay the debt. MC Breed said it best,"There ain't no future in yo' frontin'!" I will likely never get paid... no more frontin'!!