A low pass filter is a filter that only allows certain frequencies to be allowed through the filter. The low in low pass refers to it allowing frequencies below a certain cut off frequency, only allowing low frequency signals to go through. The -3db point on the filter is commonly referred to as the cutoff frequency, which is on a log scale, so any signal past this cutoff point is so small it is effectively removed. A high pass filter is the same as a low pass but only allows signals above a certain frequency. A band pass filter is obtained by putting a low pass and a high pass filter together which only allows signals below and above two different cutoff frequency to pass through effectively creating a band of frequency values that are allowed through.