A thesis indicates what the your narrowed, focussed topic is. It is still general enough that you have room to maneuver, but it puts your ideas in a box which sets limits on what can and will be discussed in the document. Probably the most important criteria for a thesis statement is that it anticipate the conclusion. Don't worry about giving too much away. Your readers need to know where the document is heading so they can put the information in the body into context.
Readers feel uncomfortable about undertaking a trip with no destination.
They use this information to decide if reading is worthwhile. The readers should be able to speculate on the predominant rhetorical purpose of the document they are about to read. Any document, especially a research essay, will probably have snippets of each purpose, but you should set out to accomplish one thing in particular. That purpose will be the predominant rhetorical purpose. Your choices are to inform, analyze, persuade or argue.