What does checking in for a flight mean?

Related questions

1. How early can I check in for a flight American Airlines?
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4. Does Qatar Airways have outlets?
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6. Is there free WIFI on Qatar Airways?
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Tags

1. #flight
2. #quantities
3. #options
4. #boarding
5. #print
6. #passengers
7. #select
8. #carrier
9. #check
10. #passes
11. #confirm
12. #their
13. #online
14. #seating
15. #specific
16. #process
17. #enter
18. #which
19. #preferred
20. #internet
21. #details
22. #baggage
23. #typically
24. #depending
25. #presence