MemoServ is part of your IRC network services, like ChanServ.
Try typing..
/msg MemoServ read 1
/msg MemoServ read 2
and so on
When you're done reading them you can type
/msg MemoServ del all to delete the memos or you can delete them individually
You must be identified with NickServ to use these commands
Also try /msg MemoServ help