RNA is a blueprint used to produce proteins in cells. A strand of RNA is composed of a string of nucleotides. Nucleotides are molecules that are like the words in a book -- when they are put together in the right order, they make a story.
Each group of three nucleotides in an RNA strand are known as a codon. The purpose of each codon is to provide the code for a single amino acid. When the right group of amino acids are formed in a sequence they fold into a protein. Codons that signal the start of a protein are known as a start codon. When all of the amino acids that are needed for a protein are accounted for, a stop codon signals the ending of an amino acid chain that will form a protein.