Page archiving is a maintenance task that is done automatically for you after all the tasks on one page have been completed.
As you complete the last active task on a page, the system will archive all the completed tasks from that page. This should be very fast. A notification banner will inform you that this is happening.
All your completed tasks that were archived can still be viewed if you switch the status to Archived (Toolbar user menu, tap the top option that's displaying the status) but they can't be edited any more.
At the moment there is no option to do this manually or switch it off.
If you'd like to get rid of archived tasks you can do so in Settings / Purge Archived.