An indel is a short polymorphism that corresponds to the addition or removal of a small number of bases in a DNA sequence. Indels are quite abundant, although not quite as abundant as SNPs. It is estimated that there are 1-2 million short indels segregating at low to high frequency in modern human populations. The vast majority of indels occur in short tandem repeats.
A 2-base deletion
Reference ACTGACACACACACTG Variant1 ACTGACACACAC--TG
A 2-base insertion
Reference ACTGACACACACAC--TG Variant1 ACTGACACACACACACTG