"runaway inflation" = "hyperinflation"
"runaway" means "out of control"
a "runaway horse" can be one that has escaped the stable OR it can mean one that is "running out of control of its rider or driver"
a "runaway car" could be one that is rolling downhill without a driver OR it can mean one that is "rolling downhill with a driver that is unable to control it"
Those two uses of "runaway" should give you an idea of what is meant by "runaway inflation". It is inflation that is increasing uncontrollably despite the efforts of the nation's government and central bank.