Your rank comes from your ELO rating in ranked games. Every account starts at 500, as a Scout, and there are seven ranks:
Every rank above Scout unlocks something new for your board, something your opponent sees, and it stays yours even if your rating later slips. Ranks unlock looks, never help: nothing that affects play is held back by rank.
Climb by winning ranked games, and see who is at the top on the leaderboard. The first four ranks also name the Quoridors-series bots, so Scout, Pathfinder, Builder and Strategist are there to practise against too.