Negative space is the space behind, around, and between an object. It represents the context of an object. It is important because it greatly influences our perception of the object itself.
You can compare the negative space with the silences between the sounds. As Claude Debussy said, “Music is the space between the sounds.”
Learning to draw the negative space is particularly important to switch our brain from the language mode to the visual mode. It forces our brain to visualize the invisible, implicit forms.