This window is used to place html-formatted text into any zone. You don't need to know HTML to put the text in. However, placing images or complex elements will need such knowledge. You can also use this window to place HTML-code of counter, soclal-media widgets etc. on your page.
Example of HTML-formatted text is shown below.
Remember that Calendar module records are very short. If you want a longer version of an event description you can always attach an article to a specific event. It can be a news article from your news feed or just a special article you wrote to make a story about what is gonna happen.