Blue cars shown on the map in the app are ones that are available to be instantly booked. Once you book, the car is yours and all you need to do to start your trip is to get to the car and follow the instructions in the app. Cars in grey are not available as of now, however, you can press "Get Notified" and receive a notification when the car is available again.
All cars also have an availability time that shows when you need to return the car by. Make sure that you return before this time to avoid late fees.