How it works
When an install is attributed, the measurement platform calls a URL provided by the network and fills in parameters such as the click ID, campaign, event name, timestamp and revenue. Install postbacks report new users; event postbacks report what those users do afterwards. The advertiser chooses which events each partner receives. Apple's SKAdNetwork uses the same word for a different flow: the device sends a signed, anonymous postback to the ad network, and a copy can go to the developer or their MMP.
Why it matters for an app team
Networks optimise on what they are told. A campaign set to find purchasers can only learn if purchase postbacks arrive, on time and with the right event names. Postbacks are also the record networks use to bill cost-per-install and cost-per-action deals.
In practice
Example: a user clicks an ad with the click ID abc123, installs the app and buys a subscription for $4.99 two days later. The network receives one install postback and one event postback, both carrying abc123, and can connect the purchase to the ad and the creative.
Common confusion
A postback is not a tracking link. The link carries data from the ad to the measurement platform before the install; the postback carries the result back afterwards. Sending a partner only its own attributed conversions is the usual setup. Sending it every conversion is a separate choice with privacy implications.