A correlation ID is a unique ID that is assigned to every transaction. So, when a transaction becomes distributed across multiple services, we can follow that transaction across different services using the logging information. The correlation ID is basically passed from service to service. All services that process that specific transaction receive the correlation ID and pass it to the next service, and so on, so that they can log any events associated with that transaction to our centralized logs. This helps us hugely when we have to visualize and understand what has happened with this transaction across different microservices.