An overview is a customizable way of displaying only relevant linked issues on the current issue. 
In addition, it's possible to edit any displayed custom fields, following the right current user's permission

You can configure one location by overview from the following list but you can configure multiple overviews for one project.

Jira Software and Jira Service Management Agent locations

Issue Glance view

Issue Panel view

Issue Activity view

Jira Service Management locations

Request details

Request panels

How to configure an overview

In the following pages, you will find the full explanation of the 3-part overview configuration.