Well, the purpose of performing a PCR (polymerase chain reaction) is to amplify (or to generate more copies of) a particular DNA sequence. The DNA sequence of interest is targeted by the addition of forward and reverse primers. Hence, the end result of PCR is that the region located in between the two primers will be amplified. PCR involves three basic steps, which include denaturation, annealing, and primer extension. During denaturation, the DNA is denatured by heat treatment, causing the double stranded DNA to separate. As the temperature is lowered, during annealing, primers bind to the DNA. Finally, during primer extension, an enzyme referred to as Taq polymerase catalyzes the attachment of complementary nucleotides, beginning from the primer. Primer extension is repeated for many cycles. Ultimately, you end up with multiple copies of the DNA sequence of interest.
I hope that answers your question.