What is it?
A challenge cache is a cache that requires the player to fulfill a geocaching goal before they are allowed to log the cache as found. Challenge caches have to be mystery caches and have the challenge attribute, they also must have the word Challenge in the cache name. Per the guidelines every challenge cache published after April 21, 2015 have to have a challenge checker.
More information about challenge caches can be found in the challenge cache guidelines at Geocaching.com.