This idiom teaches a very important lesson that ” You should not make plans that depend on something good happening before you know that it has actually happened.”
Do not assume to have everything you want until you actually have it in your hands. It’s better to wait and see the actual results before getting excited and making plans, to avoid being disappointed.
This isn’t to say that you shouldn’t be optimistic about your dream or about where you’d like to be in the future, those are both positive things. It is to say that you shouldn’t be impulsive with your decisions or hopes. Sometimes, all you need to have is a little patience and let the things happen at their own pace.