This API allows you to extract most relevant terms from a text. It is not, like many others, a basic TF-IDF analysis. It compare the text against a very large language model, it uses a probabilistic model to identify candidates, it supports multi-words terms and not only single words. It uses part of speech tagging to clean up the results". In short it is probably the most advanced term extraction out there.
This API helps you understand the text's complexity. Phrase length influence the ease of reading and understanding of a given text. Short words are usually more common (Zipf's law). Short sentences require less abstraction ability to understand. The readability analysis could be useful to make a text better, augmenting its accessibility.