A codon is a sequence of three nucleotides in DNA or messenger RNA that specifies one amino acid, or a signal to stop, during protein synthesis.
Ribosomes read mRNA three nucleotides at a time, and transfer RNA molecules bring the matching amino acid for each codon. With four bases there are 64 possible codons: 61 code for the 20 standard amino acids and 3 (UAA, UAG, UGA) are stop codons, while AUG both codes for methionine and usually marks the start of a gene.
Source: Genetic code, Wikipedia