It's been around quite a while. This article (be sure to get to the meat of the article by following link to page 2.) explains that (at least most brands) are a wash coat of thinned varnish http://www.popularwoodworking.com/article/wood_conditioner_confusion

The pre-stain conditioner is very thin varnish (shellac,
whatever, just really dilute_ - the more absorbent parts of the boards
soak up more of it, then you (typically) sand lightly and stain. The
absorbent stuff stays slightly sealed and you get even results.

" I saw on another post where a person suggested coating wood with
solvent before applying stain, which made me wonder if that's all the
wood conditioner is.??"

That may have been me, because I do it a lot ... by letting the parts of the board that are more absorbent soak up some solvent, you minimize grabbing. BUT it has the drawback of needing to be done immediately before you apply the stain, and is not as predictable as the conditioner. It works best if you also use the "wipe on thinly" stain application method and not the "slosh it on like paint and wipe it off" method.

I do it on rustic furniture because, like rustic furniture, it's fast and cheap. If I am working on something "fine", I'll use a conditioner.