Agency Plus is Upwork's paid monthly subscription for agencies, the agency counterpart of Freelancer Plus, which adds a monthly allowance of Connects and extra proposal and profile features for a fee that Upwork sets.
Agencies submit proposals with Connects drawn from the agency's own balance, and like freelancers they can buy them a bundle at a time or subscribe. Agency Plus is the subscription. Upwork's Help Center lists what it includes at any given time; the features have included a monthly Connects allowance at a lower effective price than buying them outright, the ability to see the bid range of competing proposals on a job, a custom profile URL, and keeping the agency profile visible in search through quiet periods. The price is a monthly figure set by Upwork and has changed before, so it is not restated here. It is paid from the agency's account and shows on the agency's invoices from Upwork.
For a working agency the arithmetic is simple: a team sending proposals every day burns through Connects, and the plan is worth it when the discount on the allowance alone covers the fee, before the other features count. For a small or occasional agency it rarely is.
It is often confused with two other Upwork plans. Freelancer Plus is the same idea for an individual profile and does not cover an agency's proposals; Business Plus is a client-side plan and has nothing to do with bidding. Neither Agency Plus nor any subscription changes the 10% service fee or affects the Job Success Score or badges.
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.