Accordion scheduling refers to the practice of an employer constantly adjusting the scheduling of part-time or temporary workers so as to conform the workforce as neatly as possible to demand. This prevents overstaffing and cuts down on labor costs, but it may not be much fun for the workers. Schedules can be readjusted on a daily or even hourly basis.
If you are an employer, I highly recommend it. You get what you need when you need it, and you don't have to pay people for standing around and doing nothing. If you are employee, avoid it like the devil. You never know if you'll be working any given day or hour, and your paycheck can be highly erratic from month to month.