Gain can be understood as magnification. That is, the output divided by the input. Gain is a vector, that is, it includes amplitude and phase shift. Generally speaking, pay more attention to amplitude.

When the frequency of the input signal is different, the gain of the filter is also different. For example, low-pass filter, the higher the frequency, the smaller the gain, that is, it can suppress high-frequency signals.

The gain of the bandpass filter is close to 1 in the passband, and the more it deviates from the passband, the smaller the gain is in the stopband.