EmulationStation is a frontend for emulators, giving you a graphical interface for accessing all of your favorite games. It even features support for controllers, which allows you to use the interface without needing a keyboard attached.
If you have used RetroPie on your Raspberry Pi, then you will have utilized EmulationStation before. It is a core part of RetroPie and is what lets you sort and configure your games.
Using EmulationStation on your Raspberry Pi you can start building your own alternative to RetroPie. Please note that EmulationStation doesn’t come with any emulators, so you need to set those up yourself.