Moments are timed, non-user interruptible processes within the app, such as startup, purchase, or add to cart. Like a stopwatch, the time is marked between a start and end message, both placed by the developer in the app. By tracking startup, a developer can see what's contained in the startup moment, such as long network calls, blocking calls (calls that need to finish before the startup is complete), or non-needed third party calls. Developers can observe long startups in user timelines to see where the app is held up, and can remove/modify calls in order to speed up the startup.
You can see your data related to Moments in the App Performance section of the Embrace dashboard.