What is the moral lesson taught by the fable of the tortoise?

If you mean the tortoise and the Hare by Aesop then 'slow and steady wins the race'.

The basic concept is that working slowly and methodically is better than quickly and carelessly. You'll get more done because you won't have to redo poorly done work, and more, being lazy and procrastinating have a way of catching up to you. There is also a lesson to be learned about being arrogant and assuming that someone else is less capable without actually seeing what they are capable of.