The book is about a fictional conflict between the two first mail distribution companies to exist, Thurn und Taxis (which actually existed and was the first firm to distribute postal mail) and Trystero which is a fictional invention of Pynchon's.
After being defeated by Thurn und Taxis in the 1700s, the Trystero organization goes underground and continues to exist, with its mailboxes in the least suspected places. In the plot of the novel, the existence and plans of the shadowy organization are revealed bit by bit...
... or, then again, it is possible that the Trystero does not exist at all. The novel's main character, Oedipa Maas, is buffeted back and forth between believing and not believing in them, without ever finding firm proof either way. The Trystero may be a conspiracy, it may be a practical joke, or it may simply be that Oedipa is hallucinating all the arcane references to the underground network, that she seems to be discovering on bus windows, toilet walls, etcetera.
Like in his earlier novel, V, Pynchon seems to be making a point about human beings' need for certainty, and their need to invent conspiracy theories to fill the vacuum in places where there is no certainty.