The Frontend is a collective term to name the areas of the website as visitors or registered users see it. A registered user normally works only in the frontend. It is like in a store, where the goods are displayed in shop windows and on shelves. Here you can have a look around.
So, in nutshell: it's everything an unregistered user (Guest) and all other registered users, withouth administrative user rights (the members of main Registered group and it's subgroups) can see.