To add product description for products in Shopify, you edit each product in the admin and write clear copy in the Description field

Step 1: Go to products, select product and click on “Show HTML” icon.

Step 2: To add in 2 columns, do as below. you need to add class as highlighted

Parent class name is grid and child class name is gitem wd50-lg

Step 3: Add in 3 columns

Parent class name is grid and child class name is gitem wd33-lg

3 Columns preview from demo