You can use the following ~/.fetchmailrc
poll other.mail.server protocol pop3 user "otherusername" password "otherpasswd"
where other.mail.server is the mail server you want to connect to fetch mail (usually it's your isp) and it's using pop3 protocol.
Afterwords you can run fetchmail either in daemon mode
so it polls mailserver every 300 seconds (5min), or through a cronjob