From Bulbapedia, the community-driven Pokémon encyclopedia.
A Club Master is the highest ranking member of a Club in Pokémon Trading Card Game and Pokémon Card GB2: Here Comes Team GR!. They are the Card GB counterparts to the Gym Leaders.
In Pokémon Card GB, they award Club Medals to players who defeat them in a duel as well as two booster packs.
In Pokémon Card GB 2, they give out coins, except for Rick and Mitch, who are kidnapped by Team Great Rocket when Mark/Mint first begin their quest on TCG Island. These two kidnapped Club Masters, along with the four Grand Masters, award promotional cards instead of coins as a thanks for freeing them on GR Island.