From Eterna Wiki
A codon is a group of 3 consecutive nucleotides. Any single codon corresponds to a certain amino acid or a signal to stop translation.
For the 4 canonical nucleotides in RNA - A, U, G, and C - there are a total of 64 possible codons. Out of these, 61 code for amino acids and 3 code for stop codons. This means that there are more unique codons than unique amino acids; some amino acids are coded for by multiple codons in a phenomenon called degeneracy or redundancy.