What is meant by Manual Tuning of ADF/NDB using BFO?
Manual tuning is like the old style analogue radios with a dial or rolling switch for tuning, similar to old AM/FM radios in your car, etc. Old Automatic Direction Finder (ADF) systems still have a tuning knob that does the same thing.
BFO stands for Beat Frequency Oscillator. There is a bit of electrickery going on, but basically, the ADF radio receiver processes the incoming signal and splits it in two, and just about halves one of those signal's frequencies. These two signals are then layered on top of each other so you can hear them on the headset. You then proceed to tune the radio until you minimise the pulsating sound coming from the headset.
The best analogy of how this works is how a guitarist tunes by ear by comparing two strings. When they pick two strings at the same note, if they aren't in tune there is a pulsing harmonic that doesn't sound nice. When the string is tuned properly, this pulsing goes away. Same concept is happening in your ADF radio with BFO selected.
So in summary, BFO help you tune to the exact NDB frequency using your own ear as a guide.