Use Daemon or schedule
You have 2 ways to fetch your messages, the daemon mode or the schedule mode.
In daemon mode, fetchmail process will run in memory and check accounts in it's loop.
In schedule mode each retrieval rule will be executed in independent mode at the specified schedule.
By default the fetchmail daemon run in "daemon mode".
If you enable the "Use Schedule" option, the daemon will be turned to off and each retreival rule will be executed in the specified schedule.