Best way for me to describe it is as if it were a computer OS:
Think of this, Your ROM is Windows, where the kernel is your system 32 file. You can't run windows without a system32 file and you can't run a ROM without a Kernel. It controls a lot of basic functions such as display settings, cpu speed and voltage, and sound to name a few. Installing it depends on the device, if your phone will let you flash it through twrp i would atleast wipe your caches to play it safe, if you One X was once running factory 2.20 and requires flashing through fastboot, than thats all you need to do.
Think of this, Your ROM is Windows, where the kernel is your system 32 file. You can't run windows without a system32 file and you can't run a ROM without a Kernel. It controls a lot of basic functions such as display settings, cpu speed and voltage, and sound to name a few. Installing it depends on the device, if your phone will let you flash it through twrp i would atleast wipe your caches to play it safe, if you One X was once running factory 2.20 and requires flashing through fastboot, than thats all you need to do.