A relay can have four or five terminals. They will be labelled 85, 86, 30 87 and 87A.
Terminals 85 and 86 are the coil, which when activated creates a magnetic field and pulls the contacts from the at rest position to the active position. What this means is the relay is an electrically operated switch.
Since the coil is a low power circuit, the relay can be used to switch high power with a low power control.
Terminal 30 is common to both terminals 87 and 87A. In the at rest position, 30 is connected to 87A. This is know as a normally closed contact. When the coil activates the relay, terminal 30 is no longer connected to 87A, but is now connected to 87. Terminal 87 is known as a normally open contact.
In the case of the relay 109, terminal 85 is internally connected to 30 which is connected to the battery, as you have noted, and 87A is not used. Terminal 86 connects to the ECU which grounds it when the ignition switch supplies power to part of the ECU. With battery power on 85 and ground on 86, the coil causes the contacts to move to the active position, and apply power to 87 to operate the engine controls.
Does your head hurt yet, or did this help?