I am new to audio things and I have trouble understand the waveform representation, e.g. in Audacity the y-axis goes from -1 up to +1 (please see screenshot below). What does this mean? What does the y Axis represent at all? I think it should be the loudness, but how can the loudness be negative? And don't I need multiple values at the same time on the y axis for different frequencies? Or do audio files only contain one value for a given time? I am just confused because on a spectogram one has multiple points for one point in time, on the waveform one has only one point... I already googled, but couldn't find an explanation. And help, including literature recommendations, are welcome. Thank you very much!
(I also checked the Audacity documentations on this, but it didn't help me!)