A frequent flier, technically, is someone who flies frequently. However, it has evolved into more of a specific definition, involving a traveler who usually flies with or is a member of a particular frequent flier program so as to earn rewards on that particular program.
See also: Loyalty Program.
By the way…
Flier or Flyer?
Flier is the common spelling in the United States for someone or something that flies, while a flyer is something you get handed on the street that you immediately crumple up as soon as you pass the next trash bin. In the rest of the world, flyer is common for both instances; however, it is generally accepted everywhere, including in the US, that both spellings are interchangeable.