Adding intro text, or a description, is a simple way to let your viewers know more about the lesson and prepare them for the learning experience. Intro text can be seen on the Start Lesson screen and will populate the lesson's description if you post the lesson to the Gallery.
Intro text can be added by selecting Lesson Settings from the options dropdown menu. Try to keep the description brief in order for it to fit on the start page (fewer than 180 characters) without the viewer having to scroll the text.
For more information Lesson Options, please see this article.