To use your video files with "metadata" like summaries, cast info, DVD covers, thumbnails, and more, you need to add those video files to the video library.
There are a couple of important parts to getting things imported into your Kodi library.
First, the metadata for the video file should exist someplace (the metadata are things like the title of the show, the plot description, the season and episode number if the video file happens to be an episode of a TV series, the actors, the director, cover art and episode art, etc.) The second part is naming the video files so the file gets matched up with the metadata available online.
For a TV show the scraper basically looks for the show name, season number, and episode number.
The default scrapers look at The (for TV shows) and The Movie (for Movies).
If some video file is not scraping properly, the first step is to go to the website of the scraper you are using on that source and see if the video is already listed there.
If you can't find an entry on one of those two sites for the video you're interested in, you can hunt for different scrapers which search other websites for metadata (and go through the same process of making sure your show is listed there, then naming the video files to match).
NFO files can be used to provide data for a video file in Kodi or influence the search behaviour of scrapers.
In particular they are helpful if the information fetched from a web site does not match the video file, or data for that particular file does not exist.
Media Sources are virtual links to the content you wish Kodi to use.
Alternatively, you can create new entries on those sites with some basic information about your show, and then let the scrapers do their work.
This is nice, because it means that other people will benefit from your work, and may contribute additional information about the show over time.
Adding info to a site like The is easy, but make sure you read the site rules before you start adding.
If none of that works, you can create files for each video by hand so that Kodi will bypass the normal looking online for information about your show. It's a lot of work, and it doesn't benefit anyone else the way creating entries on the metadata sites does.