How would you best describe the role of gene expression in cells?

It is the process of how a character has a gene passed from a parent, and it is then put through a probability test along with the other genes/traits. That test can be represented through a punnet square. If it is chosen through probability the character will be born with that trait if it is a physical trait it's called a phenotype. Genes or what is not seen like what's put in your DNA from your parents it is a genotype. This is the simplest definition / explanation of gene expression, I'm not sure of the set importance of it though.