I think you mean PCR (Polymerase Chain Reaction).
First, a short, complementary segment of DNA is added to one end of a portion of the DNA sequence you want to copy.
Another complementary strand of DNA is added to the other end.
These pieces are called "primers", because they provide a working place for the DNA polymerase.
Then, the DNA is heated to separate its two strands (double helix), and then cooled so the primers can bind to the single stranded DNA.
The DNA Polymerase starts making copies of the region between the primers. The copies copy themselves through many cycles of replication.
