PCR stands for Polymerase Chain Reaction, and in short, it copies DNA millions of times very quickly.
It is used in because sometimes the DNA sample is too small. This happens, for instance, in crime scene evidence, or in very old samples (eg. mummies). On the other hand, it is also vulnerable to even the slightest contamination, so it is a process that must be completed carefully.
In order to learn more about the process of PCR, I suggest going to http://learn.genetics.utah.edu/content/labs/pcr/ to perform a short PCR simulation lab.
