CoreScript is a special type of script that cannot be edited or ran by normal users. They are used by Roblox to handle built-in user interfaces found in the CoreGui among other places. They can use API that is under the RobloxScriptSecurity security level.
The source code for CoreScripts can be found within Roblox's content folder. In Windows, this is can be found at %localappdata%\Roblox\Versions\VERSION\ExtraContent\scripts\CoreScripts