A codon is a length 3 RNA string (or DNA string), which may be translated into a single amino acid during the construction of proteins. See the figure below for an illustration of a strand of RNA that has been grouped into codons.


Codons from an RNA strand are read three nucleotides at a time, and because there are three different ways in which nucleotides can be grouped into triplets, we obtain three different reading frames for protein translation.

If instead we view protein translation as induced directly from a strand of DNA, then we will have six possible reading frames: three for the strand, and three for its reverse complement.