polymerase chain reactions

Most likely the bold and underlined are enough to answer your question:

Rising temperature (94–98 °C for 20–30 seconds): causes DNA melting of the DNA template by disrupting the hydrogen bonds between complementary bases, yielding single-stranded DNA molecules.

Then the temperature must be decreased ( 50–65 °C for 20–40 seconds) in order to:  allow annealing of the primers to the single-stranded DNA template. (helps the primers to bond with the DNA so the DNA polymerase will be able to start the replication)

Then we must sightly increase the temperature because Taq polymerase has its optimum activity temperature at 75–80 °C. (commonly a temperature of 72 °C is used with this enzyme) Untill all single-stranded DNA molecules are fully extended.

And then another cycle begins by increasing the temperature at 94-98 °C


I didn't mention all details. If you want to check all the steps of PCR in more details, check this:
http://en.wikipedia.org/wiki/Polymerase_chain_reaction#Procedure
Most of my answer is copied-pasted from there. I just wanted to point out the important parts.