In general fandom terms, the One True Pairing - often abbreviated as OTP - is a romantic relationship between two characters that many fans believe is predestined, unpreventable, and/or the epitome of what a love relationship can and should be. The designation of a relationship as a OTP is not necessarily supported by canon, and canon might even refute the OTP. Fans of a particular OTP, however, could care less. They share a mildly irrational but still compelling belief that a certain couple was meant to be. In essence, that couple in the fans' minds becomes a supercouple. Since the Daria TV series has ended its run, many pairings not denied by canon are still theoretically plausible.
In Daria fandom, the most common OTPs are Daria/Trent (heterosexual) and Daria/Jane (slash). You will barely hear the term used (seriously) for other pairings, even the ones that are true pairings in canon. The former pairing is a OTP because Daria's feelings for Trent were a running plot from "The Invitation" to "Jane's Addition", the first three years of the show; the latter, because Daria and Jane's relationship is the bedrock of the series.
The phrase "One True Pairing" has an emotional subtext among adherents, bordering at times on fanaticism. A fan who has a favorite OTP can become outspoken on the topic to the point of gravely annoying other fans. A group of fans with the same OTP can generate flame wars online when the likelihood that such a OTP exists, or will ever occur, is challenged. Reality is not an obstacle. For example, Stacy/Ted is seen at times in fanfiction and has its followers, but there is nothing in canon to support it. MTV's animation department at times went out of its way to deny any possibility of a romantic or sexual relationship between Daria and Jane, but this has stopped no one from believing in it. Denial is clearly not just a river in Egypt.