Prison servers start you off in a limited map where players need to earn money to advance to the next rank and unlock the next zone. The goal is typically to earn money until you reach the top rank and unlock the freedom to access the whole map.
Money is typically earned by mining resources and farming. Ranks typically unlock new mines and allow your character to become more powerful. Typically zones will be a mix of PvP areas and safe areas so you have to becareful of your fellow prisoners.