6*His

A His tag is a tag of 6 histidines added to the end of the protein that the DNA codes for, so the DNA with contain 6 repeats of the His codon. What it is used for is purifying the protein that the DNA codes for. Histidines are positively charged amino acids and a group of 6 of them allows the protein to bind to a negatively charged column. Histidines are also special in that they have a pKa around 7, meaning that once bound to the column they can easily be unbound.