If you were to ask your dog to make a list of their top 10 favorite things in the world, and rank them, I would bet food is most of those items, including #1. Each of those items on your dog's list are worth "money" to them. For example: A piece of kibble might be worth 10 cents, a pat on the head worth a penny, but that really stinky cheese is worth $100! However, your dog chooses the value. It is their list afterall!
Each cue your dog knows has its own "Bank Account". Every time we use items on your dog's top 10 list to reward him, we put money into that behavior's Bank Account. While training a new behavior, you want to make sure you build up that account so it has a nice buffer to prevent a bankruptcy.
Once a cue has been learned to fluency, the amount of money required to reward listening can be reduced. You can use lower value rewards, such as praise or petting, and still expect your dog to perform the cue next time you ask.
If you don't replace the money in the bank account over time, the cue's account becomes overdrawn. What happens when a Bank Account becomes overdrawn? That cue will no longer work. Your dog will stop responding when asked to perform. If this continues to occur, you might poison the cue. In the case of a poisoned cue: No matter how much money is put back in, this account has gone bankrupt. A new account (a new cue) must be built up to replace it. To prevent going bankrupt and poisoning a cue, make sure you keep putting money into the account occasionally.