What is an aircraft class rating?

An aircraft class rating is a flight crew qualification that authorises the holder to operate aircraft that fit the description of the class rating (and are not designated as type-rated aircraft).
For example, a Diamond DA42 is a multi-engine aeroplane and is not designated as a type-rated aircraft, so it is covered by the multi-engine aeroplane class rating. You can find out more about aircraft class ratings on the CASA website.