Hi-Z = high impedance, Lo-Z = low impedance.
Most tabletop receivers and many portables have high and low impedance antenna inputs. The low impedance is usually around 50-75 Ohms and is normally fed with some sort of shielded cable or coax to your full wavelength sky loop on the back 40 acres of your gentleman's ranch. Hi-Z is usually around 300-500 Ohms and is normally fed with a wire hanger or that six foot long piece of extension cord we found in the dumpster outside our trailers. In radio jargon, Hi-Z also translates to "I'm broke."
Why "Z"? Radio people are
notoriously lazy sots famously industrious citizens whose use of jargon is even more
annoying sophisticated than high school kids quoting memes in real life. I've actually heard hams saying "Hi-Hi" on the radio. That's even
sadder hipper than the Sad Bachelor's mating call, "Git in the kitchen."
Take me. I can't even be bothered to write UTC half the time. So I fall back on the military "z" for zulu. So when I write "Wow, smokin' signal from WBNY at 1030z!" do I mean 1,030 Ohms? Probably, because there's no way I'd be up DXing at 5:30 am.
what do you mean by "hi-z" antenna input?