What's the difference between Description & Short Description Magento Backend. Whether we need to fill both the
column in Magento Backend panel while adding the
those are 2 product attributes. and are not connected in any way. Meaning the short description is not generated for the description.
some pages (e.g. product-detail-page) show the description and some pages (e.g. catalog-product-listing) show the short description
This is the body of the product details on the product page. Here you can put all the details that will describe your product. All the information you put there will be displayed on the details tab.
Product “Short Description”
This is up to you if you want to utilize this one. It is displayed most of the time together with the product like a unique excerpt. Typically, you can use a few words or sentences to more descriptively define the product for customers in this field.
Adding to the above answers. For every attribute, there is an option "Values Required" to control the mandatory check. If it is set to yes then the field will change to mandatory. So, it depends on the settings you have in your project.
To identify easily you will find a red asterisk mark which indicates that the filed is Mandatory. Here is a sample screenshot.
In order to change the option go to Select Stores > Attributes > Product. You will list of attributes, filter the attribute you want, select it and in the next screen you will see "Attribute Information" section > Properties > Attribute Properties change the "Values Required" option