It's when you position your speakers very, very close to you. Sort of like headphones but not, because they are in front of you. I think of it as w/i 4 ft from your seating position - but I'm not an authority.
https://www.google.com/search?q=What+is+nearfield+listening&hl=en
https://www.google.com/search?q=What+is+nearfield+listening&hl=en