Take the 4 nucleotides, A, T, C and G, and arrange them in as many combinations of 3 that you can. You'll get 64 possible combinations. This doesn't make sense until you know that several triplets can code for the same amino acid. As amino acids are formed, they link up in various ways to produce proteins. This creation of proteins is the end result of genetic expression.
Proteins serve a myriad of functions.
They also control the processes of transcription and translation. In short, a living organism develops and functions only because this vast array of proteins exists courtesy of our genes. It takes only one mistake in the process of transcribing one nucleotide from a gene on the DNA molecule to the RNA molecule to create a problem with a protein. The result might be inconsequential or it might be significant, or even devastating. This simple explanation of the gene should not mislead you into thinking that genes are simple entities.