ARPU stands for Average Revenue Per User, and is a metric that shows the average amount of revenue each active user brings to the app over a period of time. This includes revenue generated through ads, in-app purchases and subscriptions by paying and non-paying users.
ARPU is calculated by dividing the total revenue of an app by the number of active users in the same period.

Average Revenue Per User is a critical metric for app marketers because it helps them figure out which marketing strategies drive the best users. By monitoring this metric across various ad networks, channels, and campaigns, marketers can identify the best user sources and eliminate the underachievers.