What exactly is meant by 'data stored in context'?

In Go, "data stored in the context" typically refers to values associated with the context.Context object. This object is used to pass along data such as timeouts, deadlines, cancellation signals, and arbitrary key-value pairs between functions and goroutines.

For example, you can store a request identifier in the context to log it across different parts of the program:

ctx := context.WithValue(context.Background(), "requestID", "12345")

func process(ctx context.Context) {
    id := ctx.Value("requestID")
    fmt.Println("Request ID:", id)
}

process(ctx)

Thus, "data in the context" is information passed through context.Context to manage the lifecycle of operations and transmit metadata.