PCR uses primers to copy only the segment of DNA you’re looking for. Primers contain the complements of the first few “letters” of the “word” you’re looking for on your template strand (the first bases of your target sequence). They only bind to the template strand where they find those letters, and thus only those sequences are copied. Real-time PCR works by introducing fluorescent dyes and probes to the PCR process. Devices can then optically measure how much fluorescence there is in your reaction mix and use that to interpret how much target DNA was in your sample.