Gene Expression - Genome.gov
https://www.genome.gov/genetics-glossary/Gene-Expression
Gene expression is the process the cell uses to produce the molecule it needs by reading the genetic code written in the DNA. To do this, the cell interprets the genetic code, and for each group of three letters it adds one of the 20 different amino acids that are the basic units needed to build proteins.
To put it simply, genes are protein factories that make very specific kinds of protein. Gene expression is the dial for how much to ramp up (or down) production.