Second leaf, four leaf... Whaaat?


What does it mean when someone says, my fruit trees are in the second (could be the fourth, or whatever number) leaf? As an example, "This is a fourth leaf apple tree". What does this mean?
I'm sorry, but I cant find an explanation on the internet; and, I want to understand what people are referring to, when they say this phrase. I am not real experienced, at the fruit tree game, and I am looking for a little schooling.
Thanks looking, and for any responses in advance.