The term ‘negative space’ to me is confusion: technically it is the space around the main subject of a photo. It means ‘negative’ as opposed to ‘positive’ attention for the main subject. The word ‘negative’ as a noun brings back good old memories. Being older I actually worked and work with negatives (for the millennials: it has to do with film, the light sensitive stuff we used to put in a camera to get a photo on (in negative) that later could be printed (in positive).
So ‘negative space’ is about the focus a viewer of a photo is offered in a photo. If there is a lot to see around the subject, than that distracts from that subject. In other words: it is a creative tool a photographer can use.