Three nucleotides placed adjacent to each other are known as codons. A codon is set of three nucleotides that code for a specific amino acid residue.
DNA transcribed into RNA and RNA is translated into proteins. Proteins are made of chains of specific sequences of amino acids.
The order/sequence in which these amino acids are arranged is very important as it determines the structure and function of proteins. Therefore the sequence of amino acids are determined by the sequence of nucleotides of DNA/RNA which acts as the template for protein translation.
Every three nucleotides/codon will determine what amino acid to be added to the newly translated protein strand.
There are 64 codons and only 20 amino acids. therefore one amino acid has more than one codon.