Once a carter（车夫） was driving his cart, when a mantis（螳螂） jumped out in front of the cart. Raising its forelegs, it tried to obstruct the passage of the cart. Of course, it was crushed by one of the wheels.
This idiom is used to mean overrating（估计过高） oneself and trying to do what is beyond one’s ability. The result can be nothing but failure.