The primary function of the ribosome in a cell is to synthesize proteins by translating messenger RNA (mRNA) into polypeptide chains. Ribosomes read the sequence of nucleotides in mRNA and assemble amino acids in the order specified, forming proteins essential for various cellular functions. They can be found free in the cytoplasm or attached to the endoplasmic reticulum, where they participate in protein synthesis. Ribosomes are crucial for gene expression and play a key role in cellular function and metabolism.