The Free Parking allows you to claim a random card from the carousel every day for free in each city. A notification will be visible on the "Free Parking" button when your free claim is available. You can purchase more claims with diamonds.
You can also get a bonus claim by watching an ad every day.
Every time you claim a card, MONOPOLY money is added to the Free Parking coffer. You can claim all the money in the coffer if you land on the Free Parking card. A large amount of MONOPOLY money is added to the coffer when you receive either the luxury tax or income tax cards.
You can unlock the Free Parking by reaching level 3 in Atlantic City.