A "Mod Menu" in the context of Geometry Dash is usually a Dynamic Link Library (DLL) file injected into the game's process while it is running. This DLL hooks into the game's functions, allowing the user to alter variables in real-time.
Since the 2.2 update, the modding community has coalesced around several high-quality tools. These menus are typically managed through launchers like Geode, which simplifies the installation of multiple mods. Geometry Dash 2.2 Mod Menu Noclip