Answer:
Don't assume that you will get something. Wait until you actually have it.
It means to not plan something out and act as if it is going to happen, as it might not go through or it might go wrong.
It's saying don't expect the ideal conditions for something to happen in the future; like if you have many eggs don't state they will all hatch because normally the ideal condition (all the eggs growing to healthy chickens) will not occur, and a dozen eggs could produce less than 12 chicks, etc.