What do the bonus tiles mean in Words With Friends?

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Tags

1. #single
2. #lucky
3. #after
4. #extra
5. #tiles
6. #opposing
7. #equal
8. #awarded
9. #points
10. #whenever
11. #words
12. #played
13. #value
14. #friends
15. #remaining
16. #bonus
17. #player