Plain English Meaning
It is the extra rating a pilot earns to legally fly airplanes that take off and land on water. The pilot already holds a pilot certificate, and this rating is added to it after the required training and check.
Full Definition
An additional class rating added to an existing pilot certificate that authorizes the holder to act as pilot in command of an aircraft equipped to operate from water, such as a single-engine sea (ASES) or multiengine sea (AMES) airplane. It is earned by completing the required training and a practical test (or, in some cases, a proficiency check) with an authorized instructor and examiner, and is endorsed onto the pilot's existing certificate rather than issued as a new certificate.