When you create an account on MyShopify, you will be asked to name your store. Your MyShopify sub-domain will look like https://yourstorename.MyShopify.com. This is based on the store name you choose. This sub-domain is meant to be used for internal purposes, like logging into your MyShopify account and setting up and managing your online store.
You could use this sub-domain as the URL for your online store, but we don't think it's a good idea. We recommend that you use a custom domain that shows off your brand and is easy to remember to build credibility and trust.