It refers to the type of vehicle that you may operate:
CLASS A allows you to operate vehicles that tow trailers or other vehicles with a GVWR of more than 10,000 pounds. A Class “A” license also allows you to operate Class B and C vehicles.
CLASS B allows you to operate individual vehicles with a GVWR of 26,001 lbs or more OR a Gross Combined Weight Rating (GCWR) of 26,001 lbs. or more, towing trailers/vehicles rated 10,000 pounds GVWR or less. A Class “B” license also allows you to operate Class C vehicles.
CLASS C allows you to operate vehicles under 26,001 pounds. GVWR, which would not normally require a CDL; except when designed to carry 16 or more people, including the driver; or who transport 15 or fewer people (including the driver) who transport children to or from school and home on a regular basis to obtain compensation, or carry hazardous materials in quantities that require placards. It requires the corresponding endorsements.