what does the service "coregui" do?

so this is a service where im intrested because when i was looking in the developer wiki of roblox i saw something about the coregui but what is it?

CoreGui's are like the Roblox default guis that are set when a player joins the game(ex. Players and Leaderboard, Menu when you press escape, Backpack, etc etc.) You can manipulate these things using StarterGui:SetCoreGuiEnabled, hence the name (setting the core gui, or one of roblox's guis, to be enabled or disabled).

Here is the wiki for more info: https://developer.roblox.com/en-us/api-reference/class/CoreGui