The purpose of the bump or knot at the top of the Buddha's head is to signify

There are two knots famous in stories about Buddha. The first is his topknot of hair that he cut off when he crossed Lumbini. That was important because it marked his abandoning of royalty and aristocratic life and dedication to the religious ideals that he preached. It is commonly misinterpreted and mixed with the second, more important knot.

The more important knot is the Ushnisha which is like a crown of hair that is depicted as an oval on his head. It signifies his spiritual power of Buddha and his enlightenment. It is usually depicted in art as some sort of a crown made of hair that consists of infinite amounts of small curls. Historically many debate whether it actually existed or was it all just a myth regarding Buddha.