Real time PCR parameter CT
Realtime PCR uses a fluorescent dye which binds to double-stranded DNA and thus allows to measure the growing amount of DNA by each cycle. The DNA added to the reaction also binds the fluorescent dye and makes some part of the fluorescent background. When the PCR reaction starts, it takes a while until enough DNA is synthesized to go over the background and to be able to reliable distinguish the signal from the noise. I think this becomes clearer, when you look at the image below (from the NIH):



The Ct is the point where the signal can be distinguished from the background when it enters the exponential grwoth phase. The no template control stays below this threshold.