This stands for accidental insertions and deletions in the nucleotide sequence (DNA) in the genome of an organism. Such an accident can be between 1 to 10,000 nucleotides in length. In regions which code for a particular protein, such a change can have serious effects on the organism (besides the loss or gain of extra nucleotides). Unless the insertion or loss of nucleotides is a multiple of 3, it will cause a frameshift.  A frameshift means that from this point on, the reading of the triplet code will be shifted over and all the subsequent amino acids read will be read incorrectly. Apparently, each person has approximately 190 to 280 frameshifts. Since each one of us carries a heavy burden of deleterious (unfavourable or disease causing) mutations, these indels and resulting frameshifts are a major source of these problems. Most of these are not expressed in an individual because most are recessive in their effect. In this case, an individual must inherit an identical mutation from both parents (double recessive) in order to be affected.  These are not the only source of mutations, point mutations are also very prevalent.