Explain why the genetic code is called a triplet code

The genetic code is broken down into a series of codons on the mRNA. Each codon consists of 3 nucleotides which (normally) correspond to a single amino acid (e.g. AUG codes for methionine). I say normally because you can also have a codon indicate a stop codon (so no amino acid will be inserted and instead the polypeptide chain will terminate). This mRNA molecule with its list of codons will instruct a ribosome to synthesize a protein according to this code. 

This code is the set of rules - based on the triplet or codon. This triplet code is universal - all organisms use the same codons to specify the placement of each of the 20 amino acids in a protein.