The feed is a file that contains all the product information in your online store. The different pieces of information about your products are described in attributes defining each of your products. Attributes can be the name, ID, availability or the condition of the product (new, used, etc). Once you set up your account, you need to create your feed by uploading your product data. You need to decide if you are creating your feed manually using a Google spreadsheet or if you want use an extension, app, or service to auto-generate a feed. The Google Shopping app for Shopify can help you to quickly get a feed up and running. It will update your feed when you update products on your website. If you’re using another platform, you can most likely find apps or extensions that work in a similar way. The disadvantage here is that customization of your data can be sometimes a bit more time consuming with apps and extension, or even limited. If your product titles and descriptions are currently optimized on your site, it can be great to have an option like this.
In Merchant Center go to "Products", "Feeds" and press the “+”.