What NAP is
NAP stands for Name, Address, Phone. Google cross-checks your business details across dozens of directories. When they conflict - an old address here, a different phone there - Google loses confidence and ranks you lower.
How to clean it up
Pick one canonical version of your NAP. Update your website, Google Business Profile, Facebook, Yelp, and the big directories to match it exactly - down to 'St.' vs 'Street'. Fix old listings from a previous address or phone number, which silently drag you down.