The primary role of the ribosome in protein synthesis is to serve as the site where mRNA (messenger RNA) is translated into proteins. Ribosomes read the sequence of nucleotide codons in mRNA and facilitate the assembly of amino acids into polypeptide chains based on this sequence. They consist of rRNA (ribosomal RNA) and proteins and can be found free in the cytoplasm or bound to the endoplasmic reticulum, highlighting their importance in cellular function and gene expression.