The feature shows you the amount of time you spend on the app, as well as lets you set daily time limits and temporarily mute notifications.
In practice, it’s a lot like the feature that’s on YouTube, or Android 9 Pie‘s Digital Wellbeing in general. The underlying message that Instagram is trying to deliver, however, is while it’s fine to spend long hours on the app, it shouldn’t be done too often.
All that said, the feature hasn’t appeared for us at the time of writing. No idea as to when it will actually show up, but since it’s being rolled out already, maybe we’ll get in over the course of the week.