The goal with Platespotting, or Consecutive Number Plate Spotting (CNPS), is to see numbers on licence plates in consecutive order from “001” to “999” or whatever suites your countrys licence plate best. See the page Rules before you start playing the game.
If you want a bigger challenge (maybe you have finished the game once already?) then there are a few different variants of the game where you can tweak the game to make it more interesting, see the page Variants for more information.