The classifier is a match. It determines the Classification of the shooter in each DIVISION he competes in.
There are 5 Divisions and each recognizes Marksman, Sharpshooter, Expert, and Master class. The Novice class is for those unable to make the Marksman level. A sanctioned major match may or may not allow Novice Class shooters to compete.
IDPA lists shooting a classifier each year as a responsibility of each member. You can shoot local matches without being classified as the classifier is not readily available at all times. In order to shoot a Sanctioned match, you must be classified.