Travel credit can be loaded onto your card and used to pay for travel on Leap enabled services. Every time you use the card to travel the cost of the journey is deducted. If there is no credit on the card it cannot be used for any further journeys until it is Topped Up.
You can use your TFI Leap Card to travel if your card is in a positive balance, even if the balance does not cover the full fare. Your card will then go into a negative balance and will not work for subsequent journeys until topped up.