Sampling
Computing a report from a subset of the data and extrapolating, rather than from every event.
What makes it wrong.
The mechanism, not the warning.
A sampled report is labelled in the interface and easy to miss in an export or an API response. Two runs of the same sampled query can return different numbers, which is correct behaviour and reads as a bug.
In detail.
Sampling keeps large queries fast. The result is an estimate with a confidence interval rather than a count, and the wider the query, the more aggressive the sampling.
It matters most for the small numbers inside a large report: a segment representing a fraction of a percent of traffic can be estimated from very few actual events.
Where this is measured.
Sampling comes from Google Analytics 4. The server page lists every tool that returns it, with its parameters.