You earn RP for a few little things here and there but mostly for your bets paying out, as long as they aren't offset by simultaneous losses (e.g. betting Red and Black on the same roulette spin doesn't earn RP if one hits). Think of it as xp. After you earn enough lifetime RP to reach Global Tier 40 (think of it as level 40), you can choose to increase Nobility Level 0 to Nobility Level 1 and reset your Global Tier level. It's simply the prestige system.