Favourites is special folder that displays only the groups you use most often.
To mark a group as a favourite,
1. Long press the group or section
2. Press the Star icon.
Favourite groups are indicated in your data with a small star at the bottom of their icon.
You can press the star icon again to remove the favourite marker and it will no longer appear in the Favourites folder.
Only Groups and Sections can be marked in this way but any folders or cards they contain will automatically be shown in the Favourites folder.