A codon is a DNA or RNA sequence of three nucleotides (a trinucleotide) that forms a unit of genetic information encoding a particular amino acid. An anticodon is a trinucleotide sequence located at one end of a transfer RNA (tRNA) molecule, which is complementary to a corresponding codon in a messenger RNA (mRNA) sequence. Each time an amino acid is added to a growing polypeptide during protein synthesis, a tRNA anticodon pairs with its complementary codon on the mRNA molecule, ensuring that the appropriate amino acid is inserted into the polypeptide.
Okay, so an anticodon is a three-letter sequence that's complementary, or matches up, to the codon sequence found in the RNA. And the RNA, typically a messenger RNA, codes for proteins. Proteins are made up of amino acids, and these three-letter codons in the RNA are what code for the amino acids to match up with those three-letter codon sequences you need an anticodon sequence to match up with the complimentary bases. And you'll find that anticodon sequence in the transfer RNA, which is carrying the correct amino acid that it's going to attach to the protein that is being generated by that messenger RNA