Insertion, deletion of three bases/one codon or multiple of three bases/multiple codon (hence one or more amino acid) (reading frame remains unaltered from that point onwards).
Detailed answer:
Frame-shift mutations are caused by addition or deletion of nitrogenous bases in the DNA or mRNA. These mutations are so called because these shift the reading frame of codons from the site of change onwards. Deletion involves the loss of one or more nucleotides while addition or insertion involves the addition of one or more extra nucleotides in the DNA molecule resulting in shift in reading the frame of codons.
