Throughout the game you will want to retire. Retiring resets all your properties to 0 and your cash to $5,000, but it will also provide you with some Brokers based on how much money you earned. These Brokers are useful in 2 ways. First, they provide a bonus to your base income based on how many Brokers you have. Second, you can spend these Brokers on Research, which will provide huge bonuses and improvements to you Real Estate Empire. These Research effects are permanent and will stay active even as you Retire again in the future.
The retirement page shows your current number of Brokers and the number of new Brokers you will earn if you retired right then. If no new Brokers are available, it will show the amount of money you still need to earn before you are eligible for anymore. There is no point in retiring if you are not going to earn any new Brokers.