"A frameshift mutation, is like taking out a letter in this sentence: the big cat ate the fat rat. If we take out one letter, for example, the t in "ate" everything downstream changes, it becomes: the big cat aet hef atr at. Why are all of the words in the first sentence 3 letters? Remember, mRNA (which came from DNA) is "read" 3 letters at a time, that's a codon. If one of the letters goes away, everything downstream of the mutation is affected. By the same token, if one letter is added, everything downstream is also changed. This guy on Youtube does a decent job with this concept in this 5 minute video. Note that he abbreviated the amino acids, for example, he represents Proline as simply "pro". It is standard in the industry to abbreviate. In his example, line 1, proline becomes arginine after the insertion of the "G" after the first "C" in proline. It is technically an insertion mutation that causes a frameshift- everything after the amino acid "asn" is wrong!"