In-app messages are rich messages that are triggered and displayed while users are actively using your app (learn more about them here). In contrast to push messages, which are designed to alert, notify, or re-engage users, in-app messages can be used to improve a user's experience while they're in the app by providing contextually relevant information and content. You might use an in-app message to onboard new users, highlight new features or content, share coupons, or ask users to rate your app.
In-app messages can be triggered to display right at session start, or upon an event in the app. Be sure to consider the context when deciding when to display your message. Displaying a message after a user has completed a specific action in the app is a good way to ensure that your messages is relevant and doesn't disrupt a user's session in your app.
For ideas on how you can get started with in-app messaging, see this blog post featuring recommended in-app messaging use cases.