Escrow is the arrangement by which a client deposits the price of a fixed-price milestone with Upwork before work begins, and Upwork holds the money until the client approves the work or the review period runs out.
On Upwork, escrow is the foundation of every fixed-price contract. Upwork's escrow service, a licensed arm of Upwork Inc., takes the client's payment for each milestone at the moment it is funded, so the freelancer can see that the money exists before starting. The funds leave escrow in one of three ways: the client approves the submitted work and releases them; the client does nothing within the review period Upwork sets and they are released automatically; or the two sides agree a refund, in full or in part, and the money goes back to the client. A dispute freezes the funds until Upwork's mediation or an arbitration decides where they go.
That is the whole of Fixed-Price Protection: the freelancer is protected to the extent that the milestone was funded, and no further. Work done ahead of funding is done on trust. Hourly contracts do not use escrow; they rely on Hourly Payment Protection and the weekly invoice instead.
Escrow is often confused with payment. Money in escrow is not the freelancer's yet, is not shown in earnings, and can be returned to the client by agreement or by a ruling. It is also not a guarantee of quality for the client: approving a milestone is final, which is why the review period exists. Upwork states the escrow terms in its Help Center and its escrow instructions, and the review window may change.
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.