PFL stands for Pre-Fade Listen. It is a button on a mixer input channel that lets you monitor that channel - usually by sending it on its own to the headphone mix and also allowing you to view the levels on the master meters. PFL should not affect the overall mix going to the main speaker outputs. You would use PFL if you wish to monitor the levels of a channel and listen to it whilst all the channels are in use. For example, a band are playing and sound is coming into channels 1-16 on the mixer. If you hit PFL on Channel 1 then you will be able to listen to just channel 1 on the headphones in isolation.