what it means when Mail tells me "copying mails"

Hi, I just add my gmail account to Mail and finally it finishes downloading all the mails with most of which marked as "Unread" (which are already read in gmail), I just want to mark these unread mails as read so i select all the mail using Command+A then intend to mark them as read through the right click menu, now a progress bar pops up telling me "copying mails" (something like this, mine is shown in chinese), there are thousands of mails so the "copying mails" processes really slowly making the Mail kind of unreasoned then i cancel it .... and i can not mark all the mail as read since every time i select mails using Command+A, the "copying mails" window comes up.....
Can anyone tell me what the copying mails means and where these mails are copied to, and how can i mark all the mails as read?
THx a lot

Are you using Gmail via POP? You should start over and use IMAP if you want things like read state and labels to be synchronized as well...

well thanks I tried to use IMAP mode but the receive and send server seem to the same as those of POP.....and i still want to know why I can not select all the mails and mark them as Unread