There is more than one version of this fable, so I am linking the text which I used for your reference.
A mother crab and her son are out for a walk when she suddenly notices his peculiar walk. She doesn't approve of the way his toes are turned and scorns at the way he doesn't move forward in a straight line. (If you haven't seen a crab walk before, this is an important part of understanding this fable. They scuttle sideways instead of forward.)
The young crab, eager to please his mother, asks that she show him how to move forward and how to keep his toes turned "properly" by demonstrating this herself. The mother thus sets out to show her young son the way she wants him to walk. However, no matter how hard she tries, she cannot walk the way she has envisioned. When she attempts to walk forward, she keeps going sideways. When she turns her toes outward, it makes her fall over. She can only walk the way she has been designed to walk, which is exactly what she has just finished criticizing her son for.
One important lesson of this fable is that actions speak louder than words. People should therefore lead by example and should consider their own weaknesses before pointing to what they see as others' flaws.