In many multiplayer games it's possible for players to log into so-called guest profiles. If you have a friend visiting and don't want to go through the hassle of creating a new profile for him/her just to play a few rounds, guest profiles are an easy and convenient way to get the session started. Guest profiles don't have any of the usual gamer profile's functionality such as the music player and can't unlock achievements. They are named after the full profile that started the game just with a number as a suffix (ascending in case of multiple guests). Note, that players can only sign into guest profiles in games that support them (e.g. PGR4). Outside of such games, there is no way you could sign into guest profiles as a regular user as this option can't be found on the dashboard or in the Xbox Guide. But once you're signed into such a profile, you can leave the game that enabled you to sign in and start playing other games with the guest profile.