What does the "copying messages" message in the lower-left corner of the Mac Mail desktop app mean?

I would like to understand what the "copying messages" notification means in the lower-left corner of my Mac Mail desktop app? It often says something like "Copying Messages 7 of 14" and the numbers change, but not quickly. It will be set as if it is processing something for quite a while. I'm using Mac Mail to check a few Gmail accounts using IMAP.

Can someone please explain? Just to be clear, I see this message almost all the time and not necessarily when I have moved messages. And I'm certainly not copying anything on purpose.

Thanks!