What does this mean and is it a problem: openthread.thread.home.arpa

ARP is the address resolution protocol which links devices on a network at layer 2 of the OSI model.

It showing up seems to indicate you have some HomeKit "Smart" appliances advertising themselves on the network. openthread.thread.home.arpa in network , … - Apple Community

What kind of connection are you expecting while they are sleeping? What is it you need it to be connected to do while it is asleep?

I have seen some posts indicating some things "shut down" while sleeping that didn't shut down before. It's nothing I've cared about, so didn't investigate to know the full story.

See this earlier discussion.

https://discussions.apple.com/thread/253294789

It would likely depend on the context by which you found it.