The Custom Module allows you to create your own Module using a WYSIWYG (What You See Is What You Get) editor.
You can put virtually any content within the module that you like - for example: text, images, text and images.
When you want to try something such as a new module, set the access level to "Special", publish the module on a specific page of your website, and then log-in to the front-end of your website, navigate to the page where you published the module (in this case) and you will see the Module but no-one else will see it.
If/when you are happy, set the access level to, for example Public where everyone will see it.
In Joomla version 3.10.x Modules are created and managed via the administrator area menu 'Extensions' tab.