MyFav's are exercises that you have selected to be in your favourite folder. Adding an exercise to the favourite folder is done by clicking the star icon alongside the exercise name. When the star is clicked it becomes gold and the exercise will always appear in the MyFav's tab as well as in the normal library. This allows you to find your favourite exercises quickly.
To remove an exercise from MyFav's, just click the star icon again. The star icon will turn grey and the exercise will no longer appear in this tab.