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.