@Robie. Before Understanding the usage of Google Data Highlighter, I think it will be good if Know what is Structured Data? Structured Data is combination of well compiled data in way which is much liked by Search Engines and help them understanding the focus or subject of the page.
For example, if we have a page related to a valuable coin to be sold. It's quite possible that there would be more words to define the history, value, pricing and other important thing. Now if all these information will be provided in a general way.
Search engines may take it as content or article only but we know it's a product that is supposed to be sold. So we use Schema (predefined codes) to convey Search engines what this page is all about "Antique Coin". Now as Schema may be little difficult for people who are unaware of coding language hence. Google Data Highlighter is the best solution for them.
Just select the part of the page where you have mentioned all details about the coin like - value, age, color, date to sell, place to sell . Once selected a form will opened in the left hand side. Fill the information and Enter. In this way you did the same thing as schema does.
Google Data Highlighter is a kind of Structured Data tool.