It stands for the fact that you cannot have professional services delivered cheap, fast and good.
At any one time, only two sides of the triangle can exist together. You can have services delivered cheap and fast, but do not expect good. Good takes more time, resources and expertise, and they cost money. Hence, fast and good service is possible, but only with enough money paying for it. Finally, you can have services that are cheap and good, but you sure will not get it fast. Less money gets you less priority and less supporting resources.