A natural language generator (NLG) is a machine which can produce content as though it was written by a human. Natural language generators are used by some shady SEOs to produce more content with less effort.
Using a natural language generator is a terrible idea for SEO. The emphasis of your strategy should be weighted towards quality over quantity and even the best NLG on the market is no match for a decent copywriter - mainly because consistent NLGs are incredibly complex to build. NLGs are useful in more scientific fields and are useful for helping us simple humans understand big data. From a marketing perspective however, do it yourself.