What does the graph of audio signal tells? what are negative samples?
Could be a lot of things. But for typical recorded audio, it could be a graph of the change in air pressure, surrounding or in front of the microphone transducer(s), from ambient, over time, scaled by some factor. Positive samples could represent the air pressure above ambient atmospheric pressure, and negative samples the pressure below. Or both could be the opposite, as the sign can be arbitrary.
Humans (and many animals) interpret a rapid change in air pressure, within certain frequency ranges (roughly 20 to 20k Hz) and within certain (but a very wide range of) amplitude levels, as sound.