Data stream
One source of events feeding a GA4 property: a website, an Android app or an iOS app.
What makes it wrong.
The mechanism, not the warning.
A test or staging site pointed at the production stream inflates every metric with traffic that is not customers. It is common, it is invisible in aggregate reporting, and it is only findable by looking at hostnames.
In detail.
Each stream has its own measurement identifier and its own configuration, including which enhanced measurements are enabled. Events from every stream land in the same property and are reported together unless a dimension separates them.
Adding a stream therefore changes the property's totals from that day forward, with no change in behaviour.
Where this is measured.
Data stream comes from Google Analytics 4. The server page lists every tool that returns it, with its parameters.