Those two words are normally found together when you talk about accessing internals of mobile OS (operating system: Android or iOS etc.). Also, just to make it clear, it is not a phrase or an idiom. Those are two individual words.
root (You root a mobile phone by rooting process)
"Rooting is a process that allows you to attain root access to the Android operating system code (the equivalent term for Apple devices id [sic] jailbreaking). It gives you privileges to modify the software code on the device or install other software that the manufacturer wouldn’t normally allow you to." link
It is a tweak or a setting(s) which allows you to access features more than what manufacturer wants you to access.
Hope it is clear now!