Real-time polymerase chain reaction monitors fluorescence during cycling. With validated specificity, calibration and PCR efficiency, that record supports estimating initial template amount; this quantitative use is called qPCR. Ordinary endpoint PCR amplification examines accumulated product afterwards. qPCR need not start from RNA, and RT-PCR need not monitor fluorescence. Combining both operations gives RT-qPCR.[ref]
Real-Time PCR (qPCR) Basics by Thermo Fisher Scientific. Real-Time PCR Basics: amplification monitoring and quantification
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Real-time polymerase chain reaction (RT-PCR), also known as quantitative PCR (qPCR), is a laboratory technique used to amplify and simultaneously quantify a specific DNA target in a sample. It combines the amplification steps of traditional polymerase chain reaction (PCR) with the ability to measure the amount of DNA produced in real-time during the amplification process.
Also known as: Quantitative PCR (qPCR).
This describes one possible concentration detection method with fluorescent molecules that only become fluorescent when the DNA is double stranded (SYBR Green)
Polymerase Chain Reaction (PCR) - Quantitative PCR (qPCR) by Applied Biological Materials (2016)
Source. This allows you to predict the exact initial concentration by extrapolating the exponential curve backwards.