"Faves" are the files you have marked in your Library as Favorite. This is done by clicking the star icon beneath the file. Once you fave an upload, you will be able to use it in a lesson.
Click on the Library tab at top of your teacher page. Star a file you want to fave.
Once something is starred, then it becomes a favorite and can be accessed in a lesson by clicking "Choose from Favs"
Click "Add to Lesson"
...and the file will be placed in your student's lesson note.
Video, Pdf, Mp3 or photo from your library is now in a lesson!