Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Correlation is the difference between seeing many separate errors and following one business flow through the system.
A correlation id connects logs and telemetry from the same user request or business operation. The API reads or creates a correlation id at the boundary, puts it into logging scope, returns it in responses where useful, and passes it to downstream HTTP calls or queued messages. A worker then logs with the same correlation id when processing that message.