An author's thesis is their central claim or idea, expressed within a piece of writing. The body of a text serves to support this main claim, and convey the idea, with all its subtlety and nuance, to an audience.
Good writing is always a purposeful act. It is therefore natural that a text should be organized around its purpose. The author's thesis imbues the writing with such purpose and is itself the message to be conveyed, though that writing, to an audience.
In academic essays, the central claim is usually stated explicitly at the beginning of an essay, in the form of a bold assertion, the truth of which is explained through argument in the essay's paragraphs. (Since a thesis can refer to an entire research document in higher education, we often use the term thesis statement to refer to this practice.)
Below, you will find two sorces which are not academic in nature, so their theses will require a bit of thinking or introspection to identify:
In The Death of the Moth, celebrated author Virginia Woolf offers a thesis about life, and the force which drives all that live, through the struggle of one very small, yet very pure, living being. Her thesis is not stated directly at the beginning, nor anywhere else; it is implied. The reader must think for themselves in order to discover it. As you read, I hope that you will sense the the great and profound idea that is central to this short text. It is a claim that is difficult to argue plainly, which necessitates the text's prosaic style.
In this 2008 article, writer Nicholas Carr invites us to ask, Is Google Making Us Stupid? Here, the author states his thesis openly, in the third paragraph (not counting the quotation at the beginning). Relying heavily on personal experience, Carr invites us to study the way we process information in a changing age. It may inspire you to rely less heavily on AI in the future. It may also inpire you to watch Kubric's 1968 masterpiece film, as well.