gene expression
Having a gene is not the same as using it. Gene expression is the whole process by which the information in a gene is read out and turned into a working product that affects the cell. A gene sitting silent in the DNA does nothing; expression is the act of waking it up, copying it, and putting its product to work.
For a protein-coding gene the route has two main legs that you will meet again and again. First, transcription: an enzyme reads the gene's DNA and makes a matching RNA copy, the messenger RNA. Second, translation: ribosomes read that messenger RNA three letters at a time and join amino acids in the specified order to build the protein. Some genes stop at the first leg because their final product is itself a functional RNA. Crucially, expression is heavily regulated at every step: the cell controls whether transcription starts, how the RNA is processed, how long it survives, whether it is translated, and what happens to the protein afterward.
Gene expression is the reason a single genome can build a whole organism. Every cell carries the same genes, but each cell type expresses its own characteristic subset, which is why a muscle cell and a nerve cell, with identical DNA, become so different. Most of biology, development, and disease is ultimately a story about which genes are expressed, when, where, and how much, which is why measuring and controlling expression sits at the centre of modern molecular biology.
When you eat a sugary meal, certain cells switch on expression of insulin genes — transcribing the DNA into RNA and translating it into insulin protein — within minutes. The same genes go quiet again when blood sugar falls.
Expression is dynamic: genes are switched on and off in response to needs.
Not all expressed genes make protein. Many genes are expressed as functional RNAs (such as ribosomal RNA, transfer RNA, and regulatory RNAs) that never get translated; expression means producing the gene's functional product, whatever that product is.