A bonus payment is an extra, one-off amount a client sends to a freelancer or agency on an active or closed Upwork contract, outside the hourly invoice or the milestone, and subject to the usual service fee.
Upwork lets a client pay more than the contract requires. From the contract page the client can send a bonus with a note, on either an hourly contract or a fixed-price contract, and even after the contract has ended. Upwork charges the client's payment method, applies the freelancer service fee and any client fee, and the amount becomes pending earnings subject to the security period like any other payment.
Bonuses are used for the obvious reason, a thank-you for good work, and for a less obvious one: paying for something that does not fit the contract. A client who asks for a small extra task on a fixed-price contract can pay for it as a bonus rather than adding a milestone; a client whose freelancer went over the weekly hour limit can cover the uninvoiced hours with a bonus. Both are legitimate, and both keep the money on the platform.
What a bonus is not is a way to avoid fees or to pay for work outside Upwork. Fees apply in full. Nor is it a protected payment in the sense of a funded milestone: the client sends it, or does not. A bonus counts towards the freelancer's total earned, towards the client's total spend, and towards the earnings tests for Top Rated and Top Rated Plus, since Upwork measures earnings paid through the platform.
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.