The Storefront is the homepage of your system, where your clients login, and the Ordering System which they use. You can find the Settings for the Storefront in the Admin Settings pages. You can control everything about the storefront there, such as the products that are available in the storefront, the quote forms, the tax and payment process etc. You can also apply logos to it and a domain name.
Its important to understand that even though the Storefront is the whole system, some of your clients might have ‘Custom Accounts’, which is to say, they are different, and might not use any of the features of the general storefront, such as its settings or URL.