Two types of frame shift mutations are insertions and deletions. Both insertions and deletions change the sequence of nucleotides within a codon, resulting in interruption of the reading frame from which an amino acid is assembled.
Amino acids are assembled using codons that consist of nucleotides in multiples of three. The insertion or deletion of a nucleotide creates a frameshift mutation by altering the nucleotide sequence away from the original multiples of three, thus altering all codons that follow it. This leads to the sequence within the codons to encode for different amino acids. Such changes result in the assembly of incorrect or non-functional proteins.