The Siri Speech Study app allows participants to send certain data to Apple for product improvement, as detailed in the informed consent form. This app may only be used by participants who are currently enrolled in a Siri speech study. No user information is collected or used in this app except the data provided by participants.
Apple told TechCrunch the app is only being used for Siri product improvements. It offers a way for study participants to share feedback directly to Apple but there is no way for consumers to sign up, you must be invited to the study.
You can download Siri Speech Study from the App Store for free.