What is a tabiya and what is their importance? I've heard the term Tabiya used online though I've never really run into it in any chess book I've read.  I have an idea of what the term means but I'm not sure I'm correct, so first I'll say what I think it means and someone can clarify or correct me if I'm wrong.

I believe the term tabiya refers to a position that arises in an opening after a sequence of more or less standard moves from which there are multiple continuations.  That more or less I've gathered from seeing it defined online, but I believe there is a little more nuance in that the multiple possible continuations should represent distinctly different possible plans rather than just slightly different ways of trying to achieve the same plan.  So if anyone can tell me if that's correct, I'd appreciate that.

So then if that's correct, the real question is, if this is an important concept, why don't I see it mentioned much in chess literature?  Is it just because it's an obscure term and an author would probably just say something like, "From this point, White has a number of possible plans...?"
"Tabiya" is a word that came from Sanskrit and passed into Old Persian, and then to our languages. It means "battle array".

The ancestors of chess, shatranj e chaturanga, had pieces with very slow movement (for example, our "Queen", at that time called General or Advisor, could move only one square diagonally). For this reason, the opening phase of a game could be very boring, since each player had to develop pieces without really attacking the opponent. The "real" game started only when both players had pieces "close enough" to begin the battle. This "close enough" situation was called tabiya.

During the centuries, some variants of the game evolved in different ways. One lead to the "chess" we know nowadays. Another lead to some variants of Thai Chess, for example, where players use pieces with very slow movement like the original ones, but start playing directly from some well defined tabiyas and not from a standard "starting position".