These are usually single-word keywords with insane amounts of search volume and competition (for example, “insurance” or “vitamins”). Because searcher intent is all over the place (someone searching for “insurance” might be looking for a car insurance quote, a list of life insurance companies or a definition of the word), Head Terms usually don’t convert very well.
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.