The 'language' in which DNA's instructions are written. Every one of the 25,000 genes in each of our cells carries the information to make a single protein from amino acids. The sequence of bases along the gene specifies the sequence of amino acids in the protein.
However, there are only four bases, and there are twenty types of amino acid. So the bases are 'read' three at a time, to make enough combinations of three bases to specify every type of amino acid. For example, the three-base sequence 'GAA' specifies the amino acid 'glutamate'. Remarkably, the genetic code is almost identical in every organism on the planet, e.g., the same 'codon' (sequence of 3 bases) in a hamster specifies the same amino acid as it does in a human.