The optional "files" field is an array of file patterns that describes the entries to be included when your package is installed as a dependency. If the files array is omitted, everything except automatically-excluded files will be included in your publish. If you name a folder in the array, then it will also include the files inside that folder (unless they would be ignored by another rule in this section.).
'file' in package.json
Read More here:
https://docs.npmjs.com/files/package.json#files