The left side panel represents your file on your hard drive.
The right side represents a entry in MB's online database.
Scan, lookup, lookup in browser are all ways of populating the right hand panel and matching your files (you can also manually drag and drop your files from the left to right, trying to match your track to the same track on MB).
The panel at the bottom will show you old tags, and new tags (that will be applied if you hit save). What is written (and deleted) depends on your Picard settings and the information we have in our database for that track.
Clicking on the new tags values at the bottom and editing them (and then saving) will work for files on the left hand panel and the right hand panel.
It's pretty normal for items matched to our database to have heaps of new tags added - after all, that's what MB is for, all that great info! There's no such thing as 'conflicting' tags, it just overwrites old tags when you hit save.
You probably knew most of that already, but Picard's not that intuitive so thought I'd quickly go over the basics just in case!
Let us know if you have any questions re tagging or editing stuff on MB