The genetic code defines how codons, sequences of nucleotide triplets, specify which amino acid will be added next during process of translation (protein synthesis).
The genetic code is universal among organisms. That means that the same codons code for the same amino acids in different organisms.
It is known which amino acids are coded by which codons. If we know sequences of amino acids, we then also know sequences of nucleotides in DNA/RNA. If we know DNA/RNA sequences we can then reveal relationships among different organism by compering similarities in those sequences.