What is a "people mover" in a marathon?

You have 10s of thousands of people in a marathon like that.

For most of the marathon, since everyone is moving, there's not a huge problem.

As soon as people hit the finish, however, they stop, but there are still thousands of people coming in fast immediately behind them. You can get a bad logjam very quickly, and people have to still be able to get across the finish as quickly as they want.

They need to keep people moving through the lanes they likely set up at the finish. You're also going to have marathoners who are somewhat dazed and confused so they'll need a little guidance to get through, and to remember that there are others right behind them. Many when they finish are fairly self-focused on their own physical exhaustion.

You'll likely also need to keep a bit of an eye on their medical health so you can flag medics as necessary.

The second shift should still be getting fairly fast and interesting runners, many of whom will be trying to qualify for Boston, but it won't be like the first shift where you're largely standing around doing nothing since there aren't a lot of people finishing.

I'm sure the official coordinators will explain their expectations closer to or at the event. It'd be a fairly cool thing to do, and you shouldn't have to do a lot of physical work as can occur with hydration stations, cleanup, etc. Just keep your wits about you and don't screw up though. If people don't keep moving through things correctly at the end it can get ugly real fast.