A compute unit is a billing unit used by a cloud service to measure how much processing power a database or other workload uses. Instead of charging for a named machine size, the provider counts units of compute over time and bills for the units consumed. On AlloyDB Express, 1 vCPU counts as 1 compute unit.
Units make it easier to compare configurations, because a larger instance simply uses more of them. The total cost then depends on how many units you run and for how long, and on whether the service bills only while the compute is active or continuously.
A compute unit is not a universal measure. Providers define it differently, and what counts as one unit on one service may not match another. Compute is also usually only one part of the bill, since storage, backups, network traffic and optional features can be charged separately.
To estimate cost, check the provider's current pricing page for how a compute unit is defined and priced in your region, work out how many you would run and for how many hours, and add the separate charges. Re-check the figures before committing, since prices and free-tier terms can change.