Proteins are translated by reading tri-nucleotides on the mRNA strand, also known as codons, from one end of the mRNA to the other (from the 5 to the 3 end) starting with the amino acid methionine as the start (initiation) codon AUG.  Each codon is translated into a single amino acid. The code itself is considered degenerate, meaning that a particular amino acid can be specified by more than one codon. However, a shift of any number of nucleotides that is not divisible by 3 in the reading frame will cause subsequent codons to be read differently. This effectively changes the ribosomal reading frame.