Here are the various ways to affect the Meta Description for your pages:
For a Page you've created - the meta description used will be the 'summary' which you can enter within the page editor. You'll want to switch to the 'Page settings' tab and you'll find the setting in the sidebar. Note that this also applies to any page you have created, which you are currently using as your homepage.
For your home collection - by default, the homepage is your 'home' collection. In this case (where you are not using a page for your homepage), you can modify the description by editing your main shop description, which you'll find on the main Shop Settings page in your SupaDupa store's backend.
For any other collection - the Meta Description will use the description for the collection. This can be modified according to the instructions in our article on the topic.
For a product - the Meta Description will use the description for the product. This can be modified from the product edit page for the corresponding product, which can be found in the 'Products' section of your store backend.