DNA evidence is physical genetic evidence that is left behind by a person that is being accused of committing an act of some kind, normally criminal in nature.
DNA constitutes genetic markers that each person has, which identify their specific set of genes and are therefore tied to that specific person. Identical twins are able to have the same DNA, but otherwise everyone's DNA is distinct.
DNA evidence is used as evidence in criminal cases, and the presence of such evidence is enough to convict the person whose DNA was found.