According to the systemd.time, the setting
translates exactly to
ie. it activates the unit exactly at the full hour, as well as at quarter past, half past and quarter to.
Depending on your service, this may not be what you want, nor what you need in all cases.
A timer that runs every 15 minutes – for example at 1:02, 1:17, 1:32, 1:47, 2:02, … – always depending on the time it was last run – can be accomplished with the systemd.timer setting
Now, you will also want the unit to start some time after boot (unless you want to activate the unit manually or have a dependency which does that), so you should maybe specify
to have the unit started 10 minutes after boot and then every 15 minutes after that first time.
Additionally, there is the setting
OnUnitInactiveSec which starts counting the time after the service has stopped (or, more generally, the unit has inactivated).