NAP stands for Name, Address, Phone number. Data companies around the world collect and verify business data, and distribute it for clients. Firms such as Google, Yahoo, Yelp etc can subscribe to these data providers and use the information to ensure that your NAP information is consistent. Google for example will scan the web for mentions of your business name, address, phone number, website URL etc and then compares the information to other data sources.
The first thing you need to do is make absolutely sure your NAP info is correct not just on your website but on other websites on the web. For SEO results you should definitely list your business on reputable directories, for example local directories and industry specific directories.
For Google results you need to tie in your NAP info across Google+ too. All business should have consistent NAP information – it’s an important indicator of trust and you will be hampering your SEO activities if you don’t have it.