It is believed that 未 used to contain the same meaning as 末. When time passed, 未 was then used to mean something else.
To know its meaning better, let us examine its evolution history first: (image get from http://www.vividict.com)
|Learn one Chinese Character a day - 未|
So or its modern writing 未 has the meaning of not yet or in the future.
And of course, 未 contains its original meaning for the season that trees are leafy but not fruitful yet. This original meaning is not frequently used any longer.