The "First Reservation Confirmation" workflow is triggered from Core, when a new lead or trial member "Reserves" a class on the schedule.
A use case example would be this:
A client submits a rate request from your website. The Grow automated email sends them some information from the Rates workflow and indicates that the first step to get started at your gym, is to sign up for a trial. The email provides the link to a plan landing page from Core that the lead can click and sign up. After the lead signs up for the membership, they are redirected to your class schedule. If the lead reserves a class in the future, Core will send a tag to Grow called "First Class Reserved" - The addition of this tag will fire this workflow.