Hi-Z refers to an input circuit that matches “instrument level” signals that are unamplified vs. line level signals that are low impedance and are already amplified. What really matters is whether you connect instrument level signals to high Z inputs (matched) or some other type of low level input (mismatched). Plugging a line level signal into a high Z input on a preamp, for example, could over-clip or even damage the preamp circuit with overt levels of gain.
A FET input uses a “field effect transistor—FET” to boost an instrument signal, so it’s a hotter input to the first triode in a tube amp, for example. See certain Dumble-type amps that have a FET input jack, like the Bludodrive from Bludotone.