The "Free Parking" space is just free parking. Nothing happens when you land there under the rules laid out in the rulebook. But like many commonly used Monopoly rules, most of which serve to make the game take longer, people play by their own rules. The most popular for "Free Parking" is to not return fines into the bank, but to a kitty that's won by the next person to land on "Free Parking."
Hasbro recognized this after commissioning a study on how people played the game and made a "House Rules" edition. It employs many of the rules people play with that run in contrast to the actual rulebook. For instance, many don't allow players to collect rent money while they're in jail. Traditional rules say you do collect money.