what is actual digital filter doing by convolving the signal?? after convolving ,how is that input signal differ from output signal??

 

Think of it this way - convolution in time space is the same as multiplication in frequency space.

So if you have a bandpass filter and you multiply that with the frequency spectrum you only allow frequencies in the pass band.
Hence in time space, the output signal is the input signal with frequencies outside of the pass band remove.

So to generalize, all filters act on the input signal by convolution.