How does digital signal is propogated in channel?
OK you are five years old.  Have you heard the crack of lightning far away in the distance in the rain? This is a pulse with white noise of the rain. Unlike the visual signature in the image shown, we can hear over a very wide range like microscopic detail. Our ears work like many tuning forks called the cilia (which work like an fast fourier transform).  

We recognize the sound of thunder by its signature sound even if it is distorted with echos and the sound of rain. Why? Because we know how it sounds different from all other sounds, we match our expectations to the shape of this wave we hear. If it matches close enough we say we hear thunder.

So what is close enough?  It is called a "match" when most of what we hear matches what we expect only by experience. It almost sounds like a home run sound of a bat hitting a ball but different. So we remember this sound as a signature and align it with our experience.  

If we measure it's properties with frequencies, delays and amplitudes we can make a signature filter than matches the wave to optimize the signal and reduce the noise. This is called a "Matched receiver". 

It is an ideal match to this input wave in that they have the identical frequency properties so that this signal is boosted while other signatures are lowered.

So it is our experience that shapes our ability to detect this pulse of thunder and the best experience is a receiver that perfectly matches the frequency of the wave generated then conjugates the delay of the channel (air) of each frequency so they arrive undistorted, then we normalize the amplitude (AGC) and multiply it with our experience (integrating cross-correlation filter) to maximize our perception with the minimum error rate ( optimal coherent demodulation of carrier with optimal discrimination with guard bands with feed-forward error detection and correction to improve redundancy ) oops I forgot you are only 5.