What does the two dots above a letter mean in German?

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Tags

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