A superlative adjective is a groups of words used to show which item in a group of three or more has the most or least of a quality or characteristic

Usually shorter words use "the" and "est" (the nicest) and longer words use "the" and "most" (the most beautiful)

The girl sitting at the table is the nicest girl in the room.

The girl sitting at the table is the most beautiful girl in the room.

Mount Everest is the highest mountain in the world.

It is the most expensive restaurant we've ever been to.