NAICS (North American Industry Classification System) and SIC (Standard Industrial Classification) are systems for classifying companies into industries. It's a fairly antiquated standard (e.g. doesn't really account for the different classifications of modern tech companies), and we usually suggest that companies use a modern standard like our GICS based categories or the array of classification tags we return if possible.
Clearbit returns two digit NAICS and SIC codes, indicating the major group. For a full list of the codes we return please see this sheet here.