"Your main insight or idea about a text or topic, and the main proposition that your essay demonstrates. It should be true but arguable (not obviously or patently true, but one alternative among several), be limited enough in scope to be argued in a short composition and with available evidence, and get to the heart of the text or topic being analyzed (not be peripheral). It should be stated early in some form and at some point recast sharply (not just be implied), and it should govern the whole essay (not disappear in places)." -- Gordon Harvey, "Elements of the Academic Essay"

see also what to do when there's no thesis and this fuller discussion of some of the scholarly debates about the thesis statement.