The truth, the title says it all. That is indeed the topic of the movie and Clouzot shows his by the mean of this trial.
We see then almost everybody fighting for his version or perception of the truth. In fact, nobody really cares about the real truth in this trial, except maybe Dominique (and even we will see that her truth can change, through time and understanding). They fight (trial-wise) for what they stand: because they are representing the prosecution, defense, the victim ; because they are part of the "moral and virtuous", the good society or on the contrary the outcasts.
And finally through that, Clouzot manages to show his truth, the depiction of the society in these times.