An owner's representative acts on the owner's behalf to oversee a project — reviewing budgets and schedules, attending meetings, approving pay applications and change orders, and generally protecting the owner's interests. Owners hire a rep when they lack in-house construction expertise or want a dedicated advocate.
For a GC's project manager, the owner's rep is a key relationship: they're often the one approving your billings and changes and carrying your message back to the owner. It's also a common career destination for experienced PMs who want to sit on the owner's side of the table.