What is subject _INBOX.HFPwTG2g7vwhH3gXVzHjVF?

[VuongUranus]

		nats.ErrorHandler(func(_ *nats.Conn, sub *nats.Subscription, err error) {
			if err != nil {
				fmt.Sprintf("Subject: %s, Error: %s", sub.Subject, err.Error())
			}
		}),

I often get "Subject: _INBOX.HFPwTG2g7vwhH3gXVzHjVF, Error: context deadline exceeded" error, what does that mean?