As the name suggests, its the front of a store.
Storefront is basically the front end for your hybris eco system, which is visible to potential customers. They can browse products, categories offered by business. They can contact the business from here. They can order an item or cancel the purchase from here.
The storefront also allows you to search for physical stores present in your area using store locators. User can search for a product and add it his wishlist.
So basically, the store front is the control panel for the customers to get engaged with business in several ways. We normally call it the web site.