"You ain't gonna need it", or YAGNI, is a programming adage that is meant to help remind you to not overengineer.
Let's say you have to trim a tree in an arboretum. Since it's an arboretum, there are other trees. They will need to be trimmed too, eventually. Perhaps it is wise to build a tree trimming robot that will automate the task of tree trimming? If you answer yes, you are overengineering! You don't need a tree trimming robot because trees don't need to be trimmed that often. Practically, it is easier to trim the each manually, when the time is right (you might even enjoy the manual task). "You ain't gonna need it" (it being a tree trimming robot in this analogy).