On a subscriber, why is last_msg_send_time in pg_stat_subscription sometimes null?

I'm running this query:

SELECT subname, received_lsn, latest_end_lsn,
last_msg_send_time, last_msg_receipt_time
FROM pg_catalog.pg_stat_subscription;

And "last_msg_send_time" will sometimes be null. All I can find in the documentation is that this value represents: "Send time of last message received from origin WAL sender; NULL for parallel apply workers"

But I have no context for what parallel apply workers are or why they are NULL sometimes and other times not.

Can anyone explain?

Regards,

Koen De Groote