Tags allow you to classify and group the verses of your choice. As you keep on creating tags, you will be building a personalized tag list for your future use. To see all of your tags, simply visit your Bookmarks page. How to assign tags
When you click on a verse bookmark, you are greeted with Add Notes screen. This screen allows you to add notes and assign tags to the verse in subject. The green tags on this verse means they are assigned to this verse already. Gray ones are not yet assigned. You can toggle between assigning and unassigning, by simply clicking/tapping on these tags and cause them to toggle between green and gray colors to visually indicate whether they are associated with the current verse or not.