For example, I started reading a new book and want to highlight unknown words in it. So this service collects my known words and filters them from the others (unknown), then highlights new words (with possible translations).
As a result, I can create my own set of vocabulary, and keep words that I have to learn (export them via CSV or print as PDF).
After learning new words I can move them from the 'unknown' list to the 'known' list, and in the next text (that I'm reading), I will see only new 'unknown' words to continue reading.