Stock Message is the little notfication about the in-stock status of the product you have set up the stock before. The Stock Message is “In Stock” or “Out of Stock” on the storeview. Customers will check the availablity of the product through the stock message to move to the checkout process.
There are two pages you can set for the appearance of the stock message: Product Detail Page and Catalog Page. Thus, depending on your configuration whether the customers can follow in the easy way.
The stock message is displayed when you enable the
Manage Stock and set the availability of the product to “In Stock” or “Out of Stock” to Yes.
The stock message “Out of Stock” is displayed when you set the
Display Out of Stock Products and
Display product availability in stock in the frontend to Yes.
Reference: Magento 2 user guide
It comes to the end of tutorial: How to Change Stock Message in Magento 2.