An agency admin is an Upwork agency role that can edit the agency profile, manage the roster and change agency settings on the owner's behalf, without owning the agency or controlling its financial account.
Upwork describes a small set of agency roles in its Help Center, each a bundle of permissions the owner can grant to a member. The admin role is the operational one. An admin can rewrite the overview, choose directory categories and services, maintain the portfolio, invite and remove members on the roster, set their exclusive status and adjust the rate range and team size shown on the agency profile. What stays with the owner is the money: the admin cannot withdraw from the financial account or change where it pays out, and ownership itself can only be transferred by the owner.
Winning and running contracts is a different permission. That is the business manager role, which handles proposals, offers and client messages; an agency often gives one person both roles, but Upwork keeps them separate so that a marketing assistant can tidy the profile without being able to sign a contract.
From outside, nothing on the public page says who holds which role. A client dealing with an agency may be talking to an admin, a business manager or the owner, and the person named on a proposal is the member proposed for the work, not necessarily the one who sent it.
How Upwork Insights uses it
This term describes Upwork's own rules or product rather than a figure the index measures; the pages that do carry live figures are the stats and rate pages.