Maybe one of your extensions uses the functionality of another one like e.g. cron jobs. To run your extension, it is therefore required to install the cron extension, which is usually referred to as dependency. Those dependencies can be stored in the Extension Repository, so all required modules are installed automatically when you install a particular extension.
First choose the extension that you want to add as dependency and confirm your choice by clicking the "Continue" button. Then select the minimum and maximum version that is known to work with your extension.