CoreGui

The CoreGui is a service used to store Guis created in-game by Roblox for the core user interface found in every game (such as the game menu, the playerlist, the backpack, etc.). It can also be used by Plugins in Roblox Studio.

You can use the StarterGui:SetCoreGuiEnabled() and StarterGui:GetCoreGuiEnabled() methods in a LocalScript to enable and disable most elements of the CoreGui. You can also use PlayerGui:SetTopbarTransparency() to set the transparency of the top bar.