Both content and contents refer to something contained in something else. So, what’s the difference between them?

Content is uncountable; you cannot have it in the plural. It’s about the whole of something in something else.

The content of her speech really touched the audience.
He has carried out research on the fat content of frozen food.

Content providers supply material such as text, images or music, for use on websites.

In a book, content refers to all the text in a book, but contents is the list, usually at the beginning of the book, that presents the chapters of the book and what page each chapter starts at.

Obviously, contents is a countable noun – we use the plural form. We can identify the separate parts or at least understand that they are there.

He put the flask to his mouth and drank the contents.

The nouns content and contents have the stress on the first syllable.

Content pronounced with the stress on the last syllable is an adjective. This content means happy, satisfied, pleased.

He seemed very content with his new job.

Content with the stress on the last syllable can also be a verb:

I was terribly hungry but realized that I had to content myself with some wine and a small canapé or two.

Rows of small canapés with soft cheese and olives, ham, salami, etc. The image is meant to illustrate Content or contents?
A small canapé or two…