AMTuring, a library is 'a collection of media' as far as MC is concerned. All of the media that MC has imported is in your library. No matter where on your computer system the actual file resides, if it's been imported, it's in the library. A library also contains all your playlists, and customized views, etc.
It sounds like you just let MC import whatever it could find during the initial setup. this isn't always the stuff you actually want imported, and it sounds like you've got so much in your library, that you're a bit overwhelmed by it all.
As others have suggested, I think you might benefit from a fresh start.
Click on File, Library, Library Manager...
Then click on the button titled "Add Library..."
Put in a name for this new library, leave "local library" selected, then click on browse.
I suggest you browse to this location to put this new library...
C:\Users\{your name}\AppData\Roaming\J River\Media Center 18\Library\{same name as your new library}
You may have to click the "Make New Folder" button to get the last folder, in the example above it's {same name as your new library}
This puts it into a unique folder, which will be quickly filled with all the necessary items MC needs for each unique library.
Once you have created this new library, select this library from the tree to the left, then click "Load Library", which will make this new, blank library the current/active library. It will not contain any media at this point. It is your new 'blank slate'
Now, click on Tools, Import, Configure Auto-Import, then click next. This will present you with a big empty box with buttons to the right. Click on Add...
then navigate to the folder where you keep most of your Music. It will automatically import from all of the subfolders under the folder you select, unless you tell it not to. normally you want to allow it to do this.
The box below has a list of options, and these are not folder specific, they are library specific. I, personally, do not let MC "Get Cover Art", since it is sometimes incorrect, and it's much more difficult to find the incorrect cover art, then delete it and replace it, than to find missing cover art, and just get it for each album as you go along.
I suggest you let it "Analyze audio", which will determine how much 'replay gain' needs to be applied to let you play music so that all tracks play at the same relative volume (tracks don't get louder and quieter as you play them randomly). i also let it build thumbnails, Get Movie and TV info and update for external changes. however, everyone has different needs and wants here, so use the wiki for an explanation of any options you don't understand.
Also, you can always go back and check analyze audio and re-import, so anything you don't have checked now does not prevent you from doing it later. the more you have checked, the longer the import will be.
Once you've imported your music folder, then start taking a look at the different views, and smartlists, and play around with MC to start to learn how it all works. It's incredibly powerful, and can do so many things, it takes years to learn it all, and even then you still learn new stuff.
If you get to a point where you want to do something, but aren't sure how, search the forums, and the wiki, and if cna't figure it our, post specific questions, or requests, and you'll get the help you need.
Once you start to get the hang of it, you can add more folders to the auto-import and bring more of your media into the library. You can also import folders one time only, but the auto-import keeps your library up to date with any new media you acquire, so it's normally better to use the auto-import.
that should keep you busy for a little while
have fun!!