cron jobs process_membership, send_reminder and process_membership_reminder_date
The first one, process_membership, goes around and updates the membership statuses.  It's what sets a member's status to "expired" once the end date comes around.  Once upon a time, it sent reminders, but it doesn't anymore (new installations don't have that label for the job).  You should be able to have it run daily.

The second one, send_reminder, is for all reminders--activity, event, birthday, etc. in addition to memberships.  Although you could get away with running it daily, you should run this hourly, because you may have time-sensitive reminders set up for events.

Once you have the job running, you should go into the Scheduled Reminders admin page and set up the appropriate reminders.

The third one, process_membership_reminder_date, should stay disabled--it's obsolete, as the warning says.  You could even delete it if you like.