The Wanted -> Missing section contains a list of the episodes you have marked to monitor that are missing from your disk (haven't been downloaded yet).
This will only include episodes completely missing from your disk, not episodes which exist on disk, but have their cutoff profile unmet.
"Search Selected" - Here you can select certain episodes if you wish to search for them with your indexers
"Unmonitor Selected" - Here you can select certain episodes and unmonitor them if you're no longer interested in them.
"Search All" - Selecting here will send a search to all of your indexers for all current missing episodes. Once pressed, a dialog box will pop up with a warning to you, letting you know how many episodes will be searched for; this is particularly helpful to know if your indexers limit your api calls.
This search process cannot be cancelled once started without restarting Sonarr.
The Wanted -> Cutoff Unmet section contains a list of the episodes that have not yet reached their cut off. The cut off is set within your profiles
The cut off is where you essentially tell Sonarr that the quality of the video file is good enough for you and you no longer wish for Sonarr to keep searching for better.
There are a couple of options available to you on this page