[Dovecot] dovecot.conf: default_mail_env

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 ?