Custom roms have the ability to edit your phone to your needs and adjust it even better. Normally they contain custom kernels, root access, custom recovery and many other useful mods to make your user experience even better than before.
You flash the rom as a zip file in a custom recovery (such as TWRP), that will install all needed things on your phones /system partition. There are several custom roms out, like SuperMan, SuperStock, BatStock and many more!
- My mind :D