A car rental broker is an intermediary between car rental companies (Alamo, Avis, Budget, Europcar, Hertz, National, Thrifty, etc) and consumers. Car rental brokers don't own rental cars, the cars you rent are owned by the car rental companies. So you get exactly the same rental car, only for a much better price.
Renting via a broker is the united power of consumers: they turn your negotiation power into that of a big multinational company. Because car rental brokers handle large numbers of reservations (thousands a day) they are able to negotiate much lower rental prices than individual consumers or small companies could. This saves you a lot of money, because as a private customer you get the same discount as big companies that rent many cars!
Car rental companies rent most cars to businesses. These clients usually rent a car for only one day and mainly during work days. So these every-day rentals include a large amount of overhead: the car has to be rented again, the car has to be cleaned, the tank has to be filled, etc. This makes car rental expensive.
Car rental brokers rent most cars to private consumers. The average rental period is a whole week. So the car doesn't have to be cleaned every day, the tank doesn't have to be filled, the car doesn't have to be rented out again. This saves money: the longer you rent a car, the cheaper it gets. And private consumers fill the gaps in the rental schedules: they mainly rent cars during weekends and holidays, when there are no business clients. This gives brokers even more room to negotiate lower prices.
Car rental brokers also arrange for all-inclusive packages: unlimited mileage/kilometers, third party insurance, CDW insurance, theft protection insurance, airport delivery charges and local taxes are included in the quoted rental price. When you rent a car directly from a rental company, it's not unusual that what you eventually pay is twice the quoted price because the quoted price is netto. Car rental brokers offer much better rental conditions, without unpleasant surprises.