I admit that I didn't know and have never used that feature but looking at the manual I have gleaned the following and will try to explain it.
Hauptwerk has a filter installed that will affect the low frequency part of the pipe's spectrum differently than the high frequency end.
The High Frequency Boost will amplify the high frequency end if the numbers are positive and (guessing because I am not near a HW machine) negative numbers will also be possible that will attenuate the high frequency end.
Now, where does the high frequency end of the spectrum start and where does the low frequency part end? The basic setting is determined by the Transition Frequency setting. So you could adjust that so that all the harmonics will be boosted or just the higher ones.
The final part is the Transition Width. If the Transition width were zero (NB Not recommended) then the filter characteristic would look like a step. One amplitude to the left (i.e. the low frequency part of the spectrum) and another to the right (i.e. the high frequency part of the spectrum). However, the Transition Width will typically not be a step but a ramp. The change will be gradual over a range of frequencies. The final control adjusts how steep that ramp is.
I hope that helps.
Regards,