A lot like pounds, kilograms, liters, inches, or centimeters, Smiths is a measure for units of fun in a classroom setting. It is used to rate a classes' level of fun. Zero Smiths would be a unbearably boring class, while ten Smiths would be an extremely fun class.
Example: " I would rate the physics class I'm in right now ten Smiths. It's a really fun, easy class."