The concept behind highlights is to be able to easily revisit all the great experiences you had in the past. You can create highlights by tapping on the bullet icon next to your entries.
The Highlights page lets you conveniently browse through all your highlights and gives you some insights on how many highlights you have every week, month or year.
Aim for at least one highlight per week, even if it is just something very small. We need highlights in our life to stay motivated.