The mRNA leaves the cell nucleus and travels to ribosomes which exist in the cell's cytoplasm. When the RNA leaves the nucleus DNA strands resume their original double helix structure. Now, a process called translation occurs at the site of the ribosomes. This involves the translation of the RNA nucleotides into amino acids. The amino acids will eventually become a protein.
Proteins are also very large molecules.
The specific sequence of nucleotides on an RNA molecule translates into a specific protein composed of amino acids. A genetic code has been determined to decipher which nucleotides translate into which amino acids. Take the 4 nucleotides, A, T, C and G, and arrange them in as many combinations of 3 that you can. You'll get 64 possible combinations. This doesn't make sense until you know that several triplets can code for the same amino acid.