Your rank order list (ROL) is the list of all the residency programs you're willing to attend. By participating in the match, you accept a binding agreement to attend any program on that list.
As such, you must strategically approach many decisions leading up to creating your ROL to ensure you choose programs where you fit with their mission and culture as well as will be relatively happy. Then you must prioritize these programs and submit a list long enough so you're likely to match. Luckily, effective strategies are available to maximize your likelihood of matching.