A preview statement is essentially the introduction to a speech, outlining the key points that the speaker intends to cover. More than that, it serves almost as a table of contents, informing listeners of the number and ordering of a speech's points.
A preview statement's purpose is to guide the audience in their listening, and to aid in their understanding of the speech. It should be carefully delivered with short pauses between each point for clarity. Connector words like 'first,' next' and 'finally' are also useful to this end.
For instance, the speaker might say, 'first I will cover this'; 'next I will explain that'; and 'finally I will discuss such-and-such.' The preview statement will then lead in to the body of the speech, which fulfills the expectations that the speaker has set up.