Product subtitles are an option when you want to provide a short description of the product that can be used throughout the theme outside of the product page. You can save the longer description for the product page. Here's an example of a product subtitle in use on the collection page. Each olive oil has a product subtitle of short flavor description underneath the product name.
Product subtitles are metafields. Here's how to add one.
Step 1: Create a Product subtitle metafield
In the left hand column of Shopify Admin, click on the "Settings" tab towards the bottom. The settings window will open and in its left sidebar, click into Metafields. You'll see a list of Fields for different page templates. Select "Products".
Click on "Add definition". Start typing "Product" in the input field, and you should see a result for "Product subtitle". Select that and hit save. Voila! You have created a product subtitle metafield.
Step 2: Adding subtitles to products
To add a subtitle to a product, navigate to the product page in the Shopify Admin. Scroll down to the bottom of the page and in the center column, you'll see a section for "Metafields" with all the product metafields you have added to the store. Select "Product subtitle" and type in your text. Note: this description is supposed to be short and sweet so there is an automatic character limit of 70 characters.