How can an insertion or deletion mutation cause a frameshift?

In human genetics, a frameshift mutation occurs when the number of nucleotides inserted or deleted in a DNA sequence is not a multiple of three.21

In a protein-coding gene, the cellular machinery reads the genetic code in triplets called codons, which define the "reading frame" for protein synthesis. Because the code is read in these non-overlapping groups of three, the addition or subtraction of a single nucleotide—or any number not divisible by three—shifts the entire reading frame downstream from the site of the mutation.21

This shift causes every subsequent codon to be misread, resulting in the translation of an entirely different sequence of amino acids. Typically, this leads to a protein that maintains its original sequence only up to the point of the mutation, followed by a "tail" of functionally meaningless amino acids, which often results in a drastic loss of protein function.21