NAP is just your business Name, Address, and Phone number. The reason it gets its own acronym is that consistency of these three details, character for character, across every place they appear online is a real ranking factor for local search.
Google builds confidence that your business is legitimate and located where you say by cross-checking your details across your website, your Google profile, Yelp, Facebook, the BBB, industry directories, and dozens of data aggregators. When the listings disagree, "Ste 200" here and "Suite #200" there, an old phone number on one and a new one on another, that confidence erodes, and so does your ranking.
Fixing NAP is unglamorous and high-leverage: pick one exact format for your name, address, and phone, then make every listing match it. This is what "citation cleanup" means in practice.
If your site says "(414) 253-1959" but an old Yelp page says "414.253.1958" with a former address, Google can’t be sure those are the same business. Matching them is NAP consistency.
Go deeper: Why Your Milwaukee Business Isn’t Showing Up in Google Maps.
This is part of our Local SEO work.