There are a number of ways one can add movies to their library:
Manually
Use 'Scan Movie Folder'. This is useful if you have a selection of movies in a folder and you wish to orgnise them into your library. This will iterate through the movie files in the specified folder and:
If the file matches movie in your library ask whether it is a new movie, or it should replace the existing movie file
If the file does not match a movie in your library, but does match one on TMDB, offer to add the movie and copy the file into the correct place
if it's not sure ask you what to do with the file
If you already have an orgnised movie library, use bulk add to keep the current directory structure and add all movies to the library.
Copy movie files into the search folder. Enable 'Auto-add' and then run a scan. Auto add will identify that the file is new and ask to add the new movie to the library.