CFM is many cubic feet of air it pushes in a minute so higher is better. For fans I would say 20dB or below it what I consider to be silent, that's roughly the same volume as a mosquito flying around. 30 dB is probably the highest I would go, which is the same as someone whispering.
When it comes to choosing CFM I would say for 120mm 50+ CFM is good, but you would also need to consider static pressure. Just be careful when buying fans because manufactures have a tendency to put in the max CFM and the minimum dB of their fans,