Although it is often contested that the fable is attributed to Aesop. Whatever may be real source, the story is about a frog and scorpion. The lesson it tries to convey is that basic nature of a person remains unaltered however one expects to the contrary.
The scorpion needs to cross the river to go to other side but we know it does not know how to swim and is in great problem and looking for help. He chances upon a frog and begs him to give him the lift on its back and swim across the river.
Surely, the frog is apprehensive and fears the deadly sting the scorpion can impart him and kill him. So, it is hesitant but scorpion begs and begs and promises not to do anything through the journey. Finally, the frog gives in.
He hoists the scorpion on his back and the inevitable happens when they are in the middle of the course. The scorpion stings the frog sending him reeling to death. In the death throes he asks the scorpion why has he stung him as a result of which not only I will die but you will also die.
The scorpion innocently replies “I did not do it deliberately, it is in my nature to bite”. So the lesson is that basic nature is permanent and also to be careful.