Classifier is a sign that represents a class or group of objects, subjects or referents.
It also can function or act as a "pronoun" which is referred to an object or a subject. E.g. the horizontal 3-handshape representing a group of vehicles such as car, truck, bicycle, motorcycle, submarine, etc.
Classifier can be an adjective that describes a size, shape, depth, texture, etc. of a person or an object. It can be incorporated with a verb.
Classifier is one of the heaviest and the most important usages in sign language, especially in storytelling and poetry. Learning and understanding classifiers can significantly improve your expressive skills.
Once you are comfortable with using classifiers, you may find them fun to learn and play in this language.
Further research resource
Erbauch, Mary S. "Scissors, Paper, Stone: Perceptual Foundations of Noun Classifier Systems" September 1984. http://www.eric.ed.gov/PDFS/ED249787.pdf