In a paper I am working on, I come across a term called "graph canonisation "

According to math-world Wolfram :

A canonical labeling, also called a canonical form, of a graph G is a graph $G^{'}$ which is isomorphic to G and which represents the whole isomorphism class of G (Piperno 2011). The complexity class of canonical labeling is not known

Could one elaborate on that ?

Motivation : I am working on graph isomorphism (See this paper by E.M Luks).