They're members of the DC's Engineering and Facilities team or E&F (they use to be called Facility Operations or FacOps). They are exactly what their title suggests. They keep all our systems running. So things like fixing the mezzanine sorters and conveyors. Keeping miniload and roverpick operational. Keeping Inbound's ARTs running.
Basically aside from the powered equipment like forklifts (they use to fix these too, but its moved to an outside vendor) they're are responsible for fixing and maintaining all of the equipment in the building. There's 3 levels: MIT(mech-in-training), Mech 1 and Mech 2.
All of the mechs in our building have some background in industrial mechanics or electrical engineering. There use to be a test you needed to pass when you went for the position but I don't know if that's still the case with the introduction of the MIT program.