A bandpass filter is a combination of both high- and low-pass filters. This type of filter only allows a specified band of frequencies through, restricting the other frequencies surrounding the two cutoff frequencies (high and low).
Bandpass should not be confused with the term Passband, this describes the affected portion of frequencies outside of the band that was filtered through. So, a bandpass filter would have two passbands, one above and below the band of filtered frequencies.
In an ideal world, a bandpass filter would have a completely smooth passband and attenuate (remove) all of the filtered out frequencies. However, in practice, this is never the case and there is something that is known as filter roll-off, where frequencies are weakened but not removed.
The filter roll-off is shown as dB (decibels). Manufacturers continually try to design their on-board filters with the least amount of roll-off as possible, so the filter works as closely as possible to its intended purpose and keeps the sound as pure as possible.
You’ll find that synth purists will talk in detail about the quality of filters in different synthesizers and this roll-off is usually what it comes down to. The closer the filter can get to a pure passband, the happier the user and more popular that synth filter will be!