A PCR (Polymerase chain reaction machine is used to make millions of copies of a price of DNA we are interested in. Lets say you are interested in the gene that makes a bacteria turn green. you take a bit of that gene from something that is green (say plants) and you want to put that into a bacteria to make it green. What you can do is the take the gene out from the plant. But you will only get tiny amounts of DNA and for your experiment you need lots of copies of it. You can use a PCR machine to make 1000 of exact same copies from 1 copy for you to put it into the bacteria.