What do 2 dots above a letter mean?

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Tags

1. #about
2. #sound
3. #doppelg
4. #vowel
5. #indicate
6. #hungarian
7. #people
8. #refer
9. #sometimes
10. #umlaut
11. #loosely
12. #means
13. #properly
14. #these
15. #change
16. #german
17. #dieresis
18. #speaking
19. #words