It is a measure of the revenue generated by one customer phone, pager, etc., per unit time, typically per year or month.
Variations in ARPU reflect changes in the companies' ability to generate revenue and heavily impact their stock prices.
[1] That provides the company a granular view at a per user or unit basis and allows it to track revenue sources and growth.
The total revenue generated by all units (paying subscribers or communications devices) during that period is determined.
The central premise is that by attention to the margin produced per sold unit, not the amount of cash (revenue) earned from each customer, one can afford low volumes and still have a healthy company.