Keyword research is a practice search engine optimization (SEO) professionals use to find and research alternative search terms that people enter into search engines while looking for a similar subject. Search engine optimization professionals research additional keywords, which they use to achieve better rankings in search engines. Once they find a niche keyword, they expand on it to find similar keywords. Keyword suggestion tools usually aid the process, like the Google Ads Keyword Planner, which offers a thesaurus and alternative keyword suggestions or by looking into Google Suggest.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.