Okay, first things first: what exactly is the Tristero? Well, Shmoopers, here's a fictional history lesson. Ahem-hem-hem.
The Tristero has its origins in the late sixteenth century, when an outsider named Hernando Joaquin de Tristero y Calavera emerges and claims to be the rightful heir to the Thurn and Taxis Postal System. When he is denied his claim, he begins a guerrilla campaign against the current head of the system, and eventually sets up his own rival mail carrier.
Got it so far? Good, there's more.
When the French Revolution comes around, there is a rift in Tristero. Some think the Revolution is madness and want to subsidize the now defunct Thurn and Taxis System to restore order. The infighting causes the organization to fall apart, and Tristero drifts into the nineteenth century, until a number of its members immigrate to America in the 1840s.
Take a breather. There's more still.
In America, they're greeted by the Great Postal Reform of 1845, when the U.S. government declared a monopoly on mail. The Tristero resorted to yet another guerrilla campaign against U.S. Mail, attacking Pony Express postal riders on their routes and carrying out their own underground mail service. Oedipa and others suspect the group is still active today, shuttling mail back and forth between America's dispossessed.
Phew. That's a lot of fictional history for one skinny little novel.
Oh, but heads up: the Thurn and Taxis system referred to in the book actually existed under the Holy Roman Empire and continued until the eighteenth century. What Pynchon does is to weave a highly artificial conspiracy (which is 95% of the whole Tristero kerfuffle) into the historical record using fictional artifacts. Between the known facts, he places an 800-year long campaign against government monopolies on mail. (Because he's a genius like that.)
At one point, Oedipa thinks of "how tenuous it was, like a long white hair, over a century long" (4.57). Tenuous is—no doubt—the right word to describe the conspiracy, but the suggestion is that Tristero is the great underground myth. It's what has been left out of the history books. It's what "they" don't want you to know.
As a historical organization, Tristero no doubt functions as a red herring, a vast labyrinth in which Oedipa (and the lucky, lucky reader) can get lost. But Tristero also constitutes the vast symbolic network that constitutes the book. Its symbolic image—the muted post-horn of Thurn and Taxis—suggests that the traditional means of communication (like mail and, um, novels) are unreliable, and that alternative means of communication must be adopted (like weird Pynchonian postmodern novels).