A "field" is a small section of "programmable" or "dynamic" text for which Word assigns or computes a value, such as the document file name, today's date, or a page number.
Since fields are "inserted" into text, they are found in the "Insert" tab, in the "Text" section.
The use of fields can be very helpful in ordinary day-to-day business analyst documentation, for example, the very important "Table of Contents" is a field. You should be aware that they exist and how to use them.
The following document provides a complete exercise on Fields. To do the exercise, download it from the link and follow the instructions.