The HPF-Pre Series III is a high-pass filter pedal. Actually it's two filters in one: the first is set permanently at 35 Hz, and the second is adjustable from 35-140 Hz using one of the knobs on the front of the box. This gives you what its designer describes as a "12+12" curve: a 12 dB/octave drop from 35 Hz down, plus 12 db/octave drop from whatever you set the adjustable filter to. It also has a phase switch that can invert the signal. I haven't found a use for that, but it could come in handy in situations where you've split your signal and the two paths end up out of phase with each other. Lastly, the box also has an output volume control.
You might wonder why you would want a high-pass filter for a bass amp. You want a nice, heavy bass tone, so why would you cut low frequencies? One answer is that much of the low end is barely audible and can make your low end sound boomy. Trimming the lowest frequencies can tighten up your tone and actually make you sound heavier and less boomy at the same time. If you turn the adjustable filter up too high, of course you will start to lose heaviness and power, but with the adjustable filter left at a moderately low frequency, it can help a lot without loss of heaviness. (There are no markings around the filter knob, so all I can really say is that I don't hear a loss of perceived heaviness, only a progressive tightening of the low end, until the knob is turned up to about the one o'clock position. Assuming the control is linear from 35 to 140 Hz, that would probably be around 70 Hz.)
The HPF-Pre can be placed either before the amplifier or in the amp's effect/return loop. The difference between the two can be subtle, but it makes sense to me to put it in the effect/return loop so that it filters the output of the amp's tone controls.
It is powered by a single 9V battery, but it also has a 9V DC input that is compatible with typical 9V adapters and pedal board power supplies.