If you want to enable automation and and import movie and series data automatically, you will need to register for themoviedb API key and enter it in the settings page in admin area.
Create an account on https://www.themoviedb.org, if you don't have one already.
Navigate to https://www.themoviedb.org/settings/api/request
Click "Developer", then "Accept".
Select "Website" in "Type of use" field.
Fill in the requested in information in the form that appears (Can enter random values in most of the fields).
Copy API key from the next page and enter it into MTDb settings page.