Why Every real filter has some loss even at the frequencies it is supposed to pass, caused by resistive losses in its inductors, capacitors or crystals. That passband loss is called insertion loss because it is measured as the drop in signal level when the filter is inserted into the line compared with a straight-through connection. It is quoted in dB, and a good HF crystal or LC filter might show only 1 to 3 dB.
Watch out Ultimate rejection is the tempting one, but it describes attenuation far outside the passband, in the stopband, not inside it. Return loss describes how much power is reflected back due to an impedance mismatch, and Q describes the sharpness or selectivity of the response.
Insert the filter, lose some signal: insertion loss is inside the passband, ultimate rejection is outside it.