A preview statement tells your listeners, up front, the two or three main points you will cover and in what order. It usually comes at the end of the introduction, right after the thesis.
Today I will show you why the bridge was built, how it was financed, and what it means for local traffic. That sentence tells the audience exactly what is coming next.
In a two-minute self-introduction, you might say: First I'll tell you a little about where I grew up, then about my photography hobby, and finally my career goals. Those few words set up your three main points before you begin.
A thesis states your main message or position in one sentence. A preview breaks that message into the points you will cover, in order. Keep the two apart and each does its own job.
In one opening you might hear: Recycling saves our city money (the thesis), followed by I'll cover the costs, the collection process, and the results so far (the preview). the preview gives the map.