Scale to zero describes a service that pauses its compute resources when nothing is using them, so you do not pay for processing while it is idle. When a new request arrives, the service wakes the compute up and handles it. It suits workloads that are used occasionally, such as development databases, prototypes and low-traffic applications.
The trade-off is a delay on the first request after a pause. Resuming takes time, so that request can be noticeably slower than normal, which may matter for applications that need consistent response times. Some services also have limits on how often or how long they can pause.
Scaling compute to zero does not usually make the service free. Storage, backups and other components can keep accruing charges while compute is paused, and the exact billing rules depend on the provider and product. A paused database is also not the same as a deleted one, so its data and configuration remain in place.
Before relying on it, read the provider's documentation for how long the wake-up takes, what is still billed while idle, and whether the setting is available on your plan. Test the first-request delay against what your application can tolerate.