The "Module" custom field gives you the ability to choose a module out of a list which contains all of the modules of your Joomla website.
The "Module" custom field doesn't have any special characteristics as the only thing you do while creating it is to choose its Type as you can see from the screenshot below.
Once you are in your Article's Edit screen, choose the "Fields" Tab to see the Module custom field as you can see in the screenshot below.
This selectbox contains all the modules of your Joomla website and you can fast search the one you want just as you type in.
Take a look at the screenshot below to see how it could be displayed in your frontend.