A Nested polymerase chain reaction, or Nested PCR, is an alteration of the standard polymerase chain reaction with a purpose of curbing contamination. CLCSupport explains that the contamination occurs as a result of unspecified binding within products.
Such binding occurs when unexpected primer binding sites are amplified. This is referred to as mispriming. The idea behind nesting, as the National Center for Biotechnology Information explains, is to look at the outer parts of polymerase chains for large fragments. These fragments contain the expected small fragments and can be amplified without creating ambiguity. Once the outer fragments are amplified, the inner ones can then be amplified with much less contamination.