Each player has some number of Buster Points (usually abbreviated "BP"), which can be used to buy certain in-game items from the Store. Some of those items can also be bought with Rock Tokens, and some of the items available in the Store can only be bought with Rock Tokens. Big amounts of Buster Points cannot be bought with a reasonable amount of money. Instead, Buster Points are meant to be earned through finishing rounds of any game mode, similar to Experience Points.
The amount of BP rewarded from finishing a certain round depends on the game mode, the number of players (more players usually means more BP), the moment you joined the round, how well your team (or just you) performed and which Grand medals you earned personally. Playing rounds of Boss Battle with a full team of four players is usually considered to be the fastest way to earn BP among experienced players. However, for inexperienced players who have trouble beating the boss (not to mention with good grades), most other game modes are better options.