This term is often used in the world of sales. When beginning an outside sales venture, a new hire is often offered a draw against future commissions. The mistake many of them make is looking at this as a salary. While it's true that this advance on future earnings does not have to be paid back (usually) in case of failure, one only gets so many shots at a gig like this. If an employer finds out that a potential hire has exhausted his draw and failed at a couple of other outside sales businesses prior to attempting to get a gig with him, he'll usually decide against.
Here's how it works. Let's say that you and the employer both think you can earn $500 a week, at least, selling something on a straight commission basis. The employer will agree to pay you $500 a week for a specified period of time, usually not very long. Maybe 6 months. Maybe a year. The longer the guarantee, the more bullshit you can expect to have to put up with.
From then on, your results are measured against your draw. If you earn less than $500 a week in commissions for several weeks, probably due to call reluctance, you'll have some sort of sales manager down on your ass every day wondering why you're siphoning off the boss's money. On the other hand, if you are earning $1000 a week in commissions, the employer will go ahead and pay you what you're earning as long as you're outrunning the draw.
Either way, this draw is only an illusion of real money. Folks who are destined to be successful salespeople would do as well telling the employer to take their draw and stick it up their ass. I suspect this would be a great way to get someone to hire you, if you were so inclined.