This the beginning of a written work that explains what will be found in the main part

Introduction

Explanation:

The beginning of a work is called an introduction. The introduction is the section of a text in which an author first presents the ideas that will later be found in the rest of the text. The introduction helps the reader become familiar with the text, and create expectation about what is going to be discussed. Moreover, the introduction can serve as a place to introduce background information that is necessary for the reader to know before reading the text.

a caption indicating the purpose of the diagram

explanation: