PCR is a method used to amplify DNA. All you need is your DNA, primers (small pieces of DNA) complementary to two sites in your DNA (one on each strand), a heat-stable polymerase (enzyme that can make DNA according to a certain template), nucleotides and a heat-cycler. By adding all the ingredients and heating and cooling the mixture in certain preprogrammed cycles, you can amplify the DNA between the two primers.
The number of double stranded DNA pieces is doubled in each cycle, so that after n cycles you have 2^n (2 to the n:th power) copies of DNA. For example, after 10 cycles you have 1024 copies, after 20 cycles you have about one million copies, etc.