A preface is often included in reports or books by the author to explain the motivation for writing the piece or address the audience in any other way. A preface is not mandatory, but when included is put in the front matter, typically after the foreword but before any of the actual content.
There is no set formula for writing a preface. Some authors may write about the reasons that the piece is important, while others may use the space to introduce some of the most important concepts, people or characters in the piece. Some authors also write about the challenges of writing the report or make acknowledgements to those who helped.