Meaning of “first-class” - Learner’s Dictionary

first-class

adjective uk us /ˌfɜːstˈklɑːs/
BEST

relating to the best and most expensive available service, especially when travelling or sending something somewhere:

a first-class ticket
a first-class stamp
VERY GOOD

of very good quality:

It was a first-class restaurant.
first class adverb

How much is it to send this letter first class?

(Definition of “first-class” from the Cambridge Learner’s Dictionary © Cambridge University Press)