gene expression
Think of a gene as a recipe filed away in a giant cookbook. The recipe by itself does nothing; only when a cook reads it and actually prepares the dish does it affect dinner. Gene expression is that whole act of reading a gene and turning its instructions into something the cell can use — usually a working protein.
In molecular terms, expressing a gene means copying its DNA sequence into RNA (transcription) and, for most genes, then reading that RNA to build a protein (translation). Some genes are expressed only as RNA — for example RNAs that help run the cell rather than coding for protein. The amount of product a gene makes, and when it makes it, is what we usually mean when we say a gene is highly or weakly expressed.
Almost every cell in your body carries the same genome, yet a neuron and a skin cell look and behave nothing alike. The difference is which genes each one expresses. So expression is not a simple on/off fact about a gene; it is a tunable, context-dependent process, and controlling it is the heart of how one genome builds many kinds of cells.
When your eyes meet bright light, cells in the iris respond, but the underlying lens-protein genes were expressed long before — during development — to build transparent fibres that no longer make new protein.
The same genome, different genes switched on in different cells and at different times.
Expression is often measured by counting RNA molecules (for example by sequencing), but high RNA does not always mean high protein — later steps can add another layer of control.