In quantum physics, quantum coherence means that subatomic particles are able to cooperate. These subatomic waves or particles not only know about each other, but are also highly interlinked by bands of shared electromagnetic fields so that they can communicate with each other. They are like a multitude of tuning forks that are all resonating together.
As the waves get into phase, or synch, they begin to act like one giant wave and one giant subatomic particle. It becomes difficult to tell them apart.
Coherence establishes communication - like a subatomic telephone network. The better the coherence, the finer the network, and the more refined wave patterns have a telephone. The end result is a bit like a large orchestra. All the photons are playing together but, as individual instruments, each is also able to carry on playing its own part. Nevertheless, when you listen, it's difficult to pick out any one instrument.
What is even more amazing is that Popp was witnessing the highest level of quantum order, or coherence, possible in nature. Usually, this coherence - called a Bose-Einstein condensate - is only seen in material substances such as superfluids or superconductors. This is usually studied in the laboratory in very cold places - just a few degrees above absolute zero - and not in the hot and messy environment of a living thing.