Content (uncountable)
Content as a singular noun is uncountable. It means the ideas that are contained in a piece of writing or a film or a speech:
This film has adult content; it is not suitable for children.
His speech was very flowery, but it lacked content and was quite superficial.
Content can also mean the amount of a particular substance contained in something:
These hamburgers have a high fat content; they’re not good for you.
Contents (plural)
The contents of a book is the list of chapters or articles or parts that are in the book, with the number of the page they begin on:
If you look at the contents, you’ll see there’s a chapter on Japanese folk music.