IMR Leaders will have researched a given trip, they will have briefed you on what to expect when you signed up for the trip. They will provide a meeting place/time to drive to the trailhead (TH), and will have instructions on getting to the TH. For IMR purposes, the trip 'officially' begins at the TH. The Leader will likely provide some historical or comments of interest about the particular activity at the TH, to help participants understand their trip even more. The Leader likely ask the group to stay together somewhat (usually within voice or sight distance), and will provide rest stops along the way. For day trips or half-day trips, a lunch break provides a longer rest to take in views and build camaraderie.
You can expect to meet some pretty interesting people along the way, learn something about the Northern Rockies and/or desert, enjoy a challenging outdoor activity and feel good about accomplishing a goal. The trip is officially over back at the TH.