Plain English Meaning
A certificate is the main pilot license itself. A rating is an add-on to that license that says what kind of aircraft you can fly or what kind of flying you can do.
Full Definition
In FAA airman certification, a certificate is the primary credential authorizing a person to act as a pilot at a defined level (such as Student, Sport, Recreational, Private, Commercial, or Airline Transport Pilot). A rating is an authorization placed on that certificate that specifies what the holder may fly or under what conditions, including aircraft category, class, and type ratings, as well as operational ratings such as the instrument rating.
Context Anchor
Seen in practical test recommendations, where an instructor confirms that an is prepared to for a FAA approval.