ARPPU shows how much each paying member is worth. It is calculated as revenue for a period divided by the average number of payers in that period. Companies usually report it per month or per quarter.
A rising ARPPU with falling payers is a common pattern when an app raises prices or pushes members towards premium tiers. It can hide a shrinking base, so read it next to the payer count.
We use the term revenue per payer in our articles, and say "members" rather than "users", but ARPPU is the abbreviation the companies use in their results.