The polymerase chain reaction (PCR) is a technique for copying a piece of DNA a billion-fold. As the name suggests, the process creates a chain of many pieces, in this case the pieces are nucleotides, and the chain is a strand of DNA.
PCR is an enzyme-mediated reaction, and as with any enzyme, the reaction must occur at the enzyme's ideal operating temperature. The enzymes that are used for the PCR are DNA-dependent DNA polymerases (DDDP) derived from thermophilic (heat-loving) bacteria. As such, the enzymes function at higher tempertaures than the enzymes we commonly use in the laboratory or have working in our bodies. These DNA polymerases operate at 60-75°C, and can even survive at temperatures above 90°C. This is important because a part of the PCR requires that the reaction reaches ~95°C as we shall see.
Apart from the DNA polymerase, PCR needs a DNA template to copy, and a pair of short DNA sequences called oligonucleotides or "primers" to get the DNA polymerase started.