[Dovecot] dovecot.conf: default_mail_env

[Sebastien]

I just installed Dovecot on Debian and everyting seems to be working 
fine except for the fact that it can't seem to find my mails when I log 
in. All my mails are stored in the file ~/mail/Inbox. In the 
dovecot.conf file I put:

default_mail_env = mbox:~/mail/:INBOX=/var/mail/%u and it's not working.

What is the part: mbox:~/mail/ for ?
and what is the part: INBOX=/var/mail/%u for ?
I read the documentation but I don't understand. Can anyone help me ?