Each triplet “encodes” something, hence the term codon. One encodes the start of a gene and three encode the end of a gene. In the middle of a gene the codon meaning “start” (AUG) actually means something.
What do that codon and the other 60 codons mean? Each one encodes one of 20 amino acids. Every protein is a sequence of amino acids, which fold in very specific (but also very hard to predict ahead of time – – – fascinating problem in computer science.
That’s in DNA; making a protein requires copying the sequence onto RNA, where the T gets translated to U; so the start and stop codons (AUG, UAA – UGA – UAG) have T’s in them on the DNA double helix.
With 61 codons but only 20 amino acids, most of them have multiple encodings. Thus a few mutations turn out to have zero effect; all the rest change one amino acid in the resulting protein, and a few change a stop codon, meaning that the resulting protein gets big and doesn’t do its job. Or the start codon changes to something else, and the gene isn’t even findable.
Sounds scary, but life turns out to be subtle and resilient. In fact, during cell division (an adult cell is one of 100 trillion, so must descend through at least 47 cell divisions; 2 ** 47 ~ 140 trillion) every single cell division has to transcribe 2 billion codons – about 50 of them get miscopied. So after the 47th that’s ballparkish over 2 thousand miscopied codons. OUT OF 2 BILLION – no big deal. But at the same time, no two cells in your body wind up with identical DNA.
That’s the bad news; the good news is that evolution is unstoppable.