Negative space is the room's breathing room — the empty floor, wall, and air between the things you put in it. New designers tend to fill rooms; experienced designers protect the negative space because that's often what makes good rooms feel good.
The rule of thumb: every focal element needs space around it to be read. A piece of art crammed between two tall bookcases gets visually overwhelmed. The same art on a long blank wall reads as the focal point. The same piece of furniture against a busy patterned wall fights for attention; against a quiet wall, it stands out.
This is also why minimalist and Japandi rooms often photograph better than over-furnished rooms with the same number of nice pieces. The pieces are the same; the negative space lets them be seen. When a room feels "off" but you can't articulate why, the answer is often that it doesn't have enough negative space — too many things, too close together.