For a Fortnite developer, engagement refers to the amount of time players spend on your game, and how often they return to play again.
Engagement metrics focus on how often players return to an island and how long their play lasts. It is measured by daily and weekly retention, minutes played per player, number of unique players, and social play time spent in social parties.
The more player engagement your island gets, the more successful you've been in your design and execution.