If you were to load the legitimate version of the yimmenu.dll into GTA V, you would unlock a suite of tools that effectively turn the user into a "god" within the game world. These features are typically divided into several categories:
Depending on your version of GTA V, you must use the correct DLL: YimMenu/YimMenuV2: Experimental menu for GTA 5 - GitHub
In the context of software development, DLLs allow for code modularity and efficiency. However, in the context of game modification, a DLL is the preferred delivery method for "injectors." Because a DLL is designed to be loaded into memory and utilized by another process, mod developers can write a DLL that, when injected into a game’s process, begins to execute custom code.