Roblox comes with a set of GUI elements that are by default enabled for all games. These include the list of players, the current player's health bar, the player's backpack, and chat. If you do not need one of these elements or you have written your own, the StarterGui service has functionality to enable and disable each of these elements.
StarterGui has a function called SetCoreGuiEnabled which is used to turn on and off elements of the basic GUI that Roblox provides. The first thing the function needs is a CoreGuiType. This is the element you want to set. The second thing the function needs is a boolean which represents whether you want that element on or off. The list of elements you can enable or disable are:
SetCoreGuiEnabled can only be called in a local script. This means that any changes to the GUI you make will only apply to the player who owns that local script.
When a Roblox game is played on iOS, two more elements are added to the players GUI: a control pad and a jump button. Those can be hidden by using the UserInputService like this:
The Xbox One version of Roblox has a few differences than other platforms.