LT belong for the longtail, this feature is simple to understand. Let's say you run SEO Hero for the keyword « web marketing ». Into the correlated terms table, appears the word « SEO ». Clicking on the target icon for LT, you'll see every single n-gram (Keywords and/or key phrases) that contain « SEO ». For example, « SEO services », «SEO Best practices », « SEO friendly » and so on. That's a great feature to get context and keywords ideas. Some users love this feature to get untapped niche ideas.
Getting the list of the key phrases and co-occurrences you must use on your web pages is a great feature, but sometimes, the relationship between latent terms and your targeted keyword or keyphrase isn't clear. This feature allows heroes to see in which different phrases a word appears. This helps you to quickly understand the meaning(s) and the context in wich a word occur.
The HTML feature helps you to understand in which HTML tags SEO Hero found a keyword and how many times it found it within this tag. That's another way to understand how important is a word on a given topic, and to know how competitors are optimizing it.
DF belong for domain frequency. It indicates (in percents) on how many web pages a term was used. Let's say that you run SEO Hero on the query « ranking on Google » and the term « SEO » shows a DF = 92. That simply means that SEO was found on 92% of the web pages that rank in the top 100 for that query and that SEO is a very important keyword to add in your content.
TF belong for term frequency. When SEO Hero find a word in one page it counts how many times the word appears on that page and how many differents words occurs on the same page. So SEO Hero can compute the term frequency or keyword density. When SEO Hero explore another web page and find the same term, it calculates it term frequency at the page level, and makes the mean of all data.