ARPDAU tells you how much a typical active player generated on a given day, across ads, in-app purchases and subscriptions combined. Because it normalises for audience size, it is the metric teams use to compare days, markets and versions.
It is also an aggregate, not an explanation. ARPDAU can rise because ad yield improved, because payer conversion moved, because a sale pulled purchases forward, or simply because the mix of countries or platforms changed. Before acting on a movement, split it into ad revenue per DAU and IAP revenue per DAU, then by market and platform.
A common trap is reading a monetization change on ARPDAU alone: revenue per user can rise in the first week while retention, and therefore total revenue, falls later.