If you've ever had one of those cards from a coffee place where every time you buy a cup they punch it, and when you get ten punches you get a free cup, you know what a frequent flyer program is. You get a reward after some number of purchases in order to convince you to get your coffee (or plane tickets) there rather than from a competitor. The rest is in the implementation details. As noted, airline programs tend to be a bit more complicated than the coffee cards, and they vary from one airline to the next, but the principle is exactly the same.