Optional field in admin panel to filter nodes in view
Not a problem... Drupal does have its own jargon but after a while you will get it.
Very simple solution whenever you want to offer user an option of refining the view or sort... Expose the relevant filter(s), in this case the vocabulary or taxonomy term... 
You might also think about using the global null function so as to reduce load on your server ... This will basically produce no results until the user chooses a vocabulary or taxonomy term (category)... Here is an excellent walkthrough on how to do this: How to prevent views from loading the results upfront

Cheers,
gg