The ribosome reads an mRNA in codon (3 consecutive nucleotides) sequences. It begins with a start codon that codes for methionine. As the ribosome reads the codons, the respective tRNA (with the respective anticodon) is recruited bringing in an amino acid coded for by the codon. The amino acid brought in then forms a peptide bond (mediated by peptidyl transferase enzyme) with the preceding amino acid. This continues until the whole mRNA is read or until a stop codon is encountered. A polypeptide will eventually have been formed.