A band-pass filter will let only frequencies pass that are in a previously specified range. For example, in the atmospheric sciences it is common to band-pass filter the data with a period range of, say 3 to 10 days, so that only cyclones remain as fluctuations in the fields.
A bandpass filter is an electronic circuit that lets through frequencies between two given frequencies. For example, an ideal bandpass filter might let through all signals above 30Hertz but below 100Hz. All of the signal outside this range is attenuated.
In practice, most bandpass filters do not attenuate frequencies just outside the desired frequency range completely.
See also band-stop filter.