AUG is the usual start codon on messenger RNA.
During translation, a ribosome generally begins reading a protein-coding sequence at an AUG codon. AUG specifies methionine in standard genetic code, so newly made proteins usually begin with methionine, although that residue may later be removed or chemically modified.
The start codon does more than specify an amino acid: it establishes the reading frame for the codons that follow. In bacteria, an upstream ribosome-binding sequence helps position the ribosome, while eukaryotic ribosomes commonly scan from the 5′ end to find a suitable AUG context.
AUG is not the only possible initiator in every organism. Some bacteria can use alternative start codons such as GUG or UUG, but AUG is the canonical answer. UAA, UAG, and UGA are stop codons rather than start signals.