Properties have several stats: max rent, size, wealth, and color group bonus.
Size is the amount of individual tiles that can host buildings, like houses, hotels, businesses, and decorations. It does not change over time or with the property’s level.
Wealth is the value of the property. It increases with the property’s level.
Max rent is the most important statistic of a property is a cap called max rent. The property accumulates the monopoly money from the rent of residential buildings placed on it, until the cap is reached. As a player, you can collect this money pretty much at any point in time, but be aware that after the cap is reached, you cannot accumulate more. You have to come back to the game regularly in order to maximize how much money you make from rents.
Color group bonus is either locked or obtained. You need to open for business all 2 or 3 properties belonging to a color group (ex: Mediterranean and Baltic form the initial brown color group), in order to unlock the group bonus. See below how this works.