Active threads waiting on IPC / messagequeuesend

I'm looking at a pg_stat_activity dump where there are a group of active threads stuck for a long time in MessageQueueSend

7:52.394089+00 | IPC             | MessageQueueSend | active |             |     27406510 | -5892847938856697008 | SELECT ....

Does this mean they are blocked on clients reading data or does it represent something blocking on the server side?

Thanks

Ben