The codon CUG specifies the amino acid leucine. In the genetic code, codons are sequences of three nucleotides that correspond to specific amino acids or stop signals during protein synthesis.
CUG is one of the codons that codes for leucine, which is an important amino acid in many proteins. The genetic code is almost universal and redundancy in the code means that several codons can specify the same amino acid, but CUG does specifically point to leucine in the context of protein synthesis.