Wikipedia defines a Challenge coin as, "A small coin or medallion, bearing an organization’s insignia or emblem and carried by the organization’s members. Traditionally, they are given to prove membership when challenged and to enhance morale."
They are sold in recognition of the special achievement of reaching the summit of a Colorado 14'er. When you receive one of these Challenge Coins you know you have become a part of something bigger.
Challenge coins come in either Standard, Mount Bierstadt, Longs Peak, and the Special Edition: Support Law Enforcement.