It means a system capable of producing the full audiable spectrum (20hz to20KHz) and be flat accross that range (so no peaks or troughs at certain frequencies).
Yeah I had to look it up earlier too. I just started on tgp recently and realized after almost 25 years of playing I don't know **** about gear. Learning pretty quickly now though!
It is a typical example of what we mean when we talk about FRFR cabs and a fairly high quality cab. It is important to realize though that when we talk about FRFR cabs, none of them, including the QSC, are really flat response. That is more of an aspiration than an actual characteristic. They are much more flat response than a typical guitar cab but they all have their own characteristic response variations.
The QSC K12 is a popular one, but whether it's a good one or not I cannot say. I have read a lot of posts on this forum from people who greatly prefer the K10 over the K12 for use with a guitar and modeler.
I use a FRFR monitor with my Axe-FX II (either the FBT 8ma or the RCF NX12 SMA depending on how much volume I need). It definitely does not sound the same as using a guitar amp, but I like the results. It's much more like the sound you would get by micing your amp and listening to it through studio monitors.