Developers are now receiving emails letting them now about the availability of the new feature, which is free with an iOS Developer Program membership and requires no additional coding or app updates.
App Analytics gives developers access to a number of different usage stats, letting them determine the effectiveness of marketing campaigns and promotions. The feature lets developers see which websites and blogs drive the most traffic to an app, and it determines how many app downloads a developer's App Store product page earns.
By creating unique links for each marketing campaign, a developer can determine what's effective and what's not, and detailed user engagement metrics let developers see how people are using apps. There's information active devices, number of sessions, and retention, so a developer can tell how long someone's using an app and how often.
Apple's App Analytics information is currently available to all iTunes Connect users who have an Admin, Finance, or Sales role. On the consumer end, customers need not worry about developers having access to personal info -- Apple only displays aggregated data, not personally identifiable information from customers. Users also need to opt-in to share their app usage data to Apple, which can be done via Settings --> Privacy.