WHAT IS ARCHIVING MAIL OPTION IN GMAIL?

Archiving moves messages out of your inbox and into All Mail, letting
you tidy up your inbox without deleting anything. Any message you've
archived can be found in All Mail, in any labels you've applied to it,
and in Gmail search results. When someone responds to a message you've
archived, the conversation containing that message will reappear in
your inbox.

Source:
http://mail.google.com/support/bin/answer.py?hl=en&answer=6576