Noob question!
What does that do? And what's a Rom?
A ROM is the operating system but because Android is open source it can be customized by other developers outside of ASUS.
In order to install a ROM that isn't from ASUS you need to first unlock the bootloader which is what the tool does that ASUS released today.
Then once that's unlock you can install a custom recovery which is the tool used to backup and install other ROMs.
The reason people install custom ROMs is because it gives you better control over the device and can have a completely different look and feel or additional speed tweaks and over clocking.
It's a bit more complicated than that but that's the basic idea.
One thing to note that unlocking your bootloader will void your ASUS warranty so make sure you are good with that before proceeding
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