Negative space is the "white space" --not necessarily white, but it is often a neutral-- the area without design. If you look at a newspaper sales flyer, generally there is very little white space. If you look at a beautifully printed book of poetry, sometimes there will be far more whitespace than print. Using white space indicates that the design work is important and deserves your undivided attention. It takes a lot of nerve to use white space lavishly, but the effect can be very dramatic and graphic.
RST
When my "papa" (grandfather-- died over 40 years ago), ran for office back when, he bought a full page ad and put a few small words in the middle of the page with his name, etc. That's an attention-getter!