What does the saying "The future doesn't exist because the future is the past" mean?
The future does not exist because it has not happened yet, you can only imagine what will occur. When you reach the time that you will call the future, it is already now the present and once you are in present time, it is now the past.
For example: if the future is one minute from now. Once that minute arrives, you are in the present moment, once that minute changes to the next it is now in the past. The future is only something that can be thought of, but not actually experienced.