What does Macintosh HD not mounted mean?

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Tags

1. #operating
2. #showing
3. #accessible
4. #mounting
5. #select
6. #write
7. #utility
8. #operations
9. #fixed
10. #drive
11. #macintosh
12. #sierra
13. #perform
14. #click
15. #mounted
16. #mount
17. #means
18. #available
19. #system