[Usenet, GEnie, CompuServe] Common abbreviation of `topic thread', a more or less continuous chain of postings on a single topic. To `follow a thread' is to read a series of Usenet postings sharing a common subject or (more correctly) which are connected by Reference headers. The better newsreaders can present news in thread order automatically. Not to be confused with the techspeak sense of `thread', e.g. a lightweight process.
Interestingly, this is far from a neologism. The OED says: "That which connects the successive points in anything, esp. a narrative, train of thought, or the like; the sequence of events or ideas continuing throughout the whole course of anything;" Citations are given going back to 1642!