Hi Lisa, Filtering a signal usually implies convolving the signal with the impulse response of a linear time-invariant system.
In the frequency domain this is equivalent to mulitplying the frequency response of the signal with the frequency response of the filter.
Accordingly, it is desirable to have the magnitude of the filter as close to 1 as possible in the passband, so that in the passband, the frequency content of the signal is not modified in magnitude, just in phase.
