I apologize for the confusion, allow me to explain the joke to you. The joke is based on a set-up and punchline. In the set-up, a man is being threatened by robbers who want to take his valuables. Instead of giving in to their demands, the man appears confident and ready to defend himself and his pet turtle.
However, when the robbers express doubt about the turtle's ability to help, the punchline of the joke is delivered. Rather than confronting them, the man counters with a joking reference to the turtle's speed and agility, implying that the robbers are slow and inept.
The humor in the joke is derived from the unexpected nature of the response, as well as the silly and exaggerated image of a turtle outrunning a group of robbers. I hope that clears things up!