Each cue your dog knows has its own "Bank Account". Sit has an account, Down has an account, Come has an account, etc. Accounts with more money in them are more likely to produce responses when cued. When we are first starting training we want to put a bunch of money in each account so we have a buffer for the future when we don't add money as often.
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 after all!
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. Have enough money in the account and you can expect your dog to respond in the future, even when they haven't been paid this time.
Working in new environment and around bigger distractions also cost money for our dog. Your dog might sit every time you ask in your kitchen, but you ask for a sit at the dog park while their best friends are playing behind them and they fail to respond. This means your sit account might not be as big as you thought. Take enough money out of an account, and it becomes overdrawn. When a behavior is overdrawn, your dog will not respond to the cue.
Some behaviors are "cheap" behaviors. These are behaviors that your dog is likely to offer, even with there are distractions around. Even cheap behaviors can become overdrawn and go bankrupt, so don't forget to reinforce them and add money back into their accounts.
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.