When biologists talk about a "universal genetic code," they are referring to the fact that in nearly all organisms, the same codons (sequences of three nucleotides) code for the same amino acids.

Here's what this means:

* DNA and RNA: All living things use DNA as their primary genetic material. This DNA is transcribed into RNA, which then serves as a template for protein synthesis.

* Codons: The sequence of DNA (or RNA) is read in groups of three nucleotides called codons. Each codon corresponds to a specific amino acid.

* Amino Acids: Amino acids are the building blocks of proteins.

* Universality: The same codons code for the same amino acids in nearly all organisms. This means that, for example, the codon "AUG" will always code for the amino acid methionine, regardless of whether you're looking at a human, a bacterium, or a plant.

There are a few exceptions to this universality:

* Mitochondria: These organelles within cells have their own DNA and a slightly different genetic code.

* Some bacteria and archaea: These single-celled organisms have a few codons that code for different amino acids than in the standard genetic code.

The near universality of the genetic code is a powerful piece of evidence supporting the idea that all life on Earth shares a common ancestor. It suggests that the fundamental mechanisms of life, including the way genetic information is stored and translated, were established very early in the history of life on Earth.