It's depicting the Trial of Phryne, which tookplace in Athens sometime during the fourth century BC. Supposedly, Phryne was accused of impiety. As it became apparent that the judges were going to give a guilty verdict, Hypereides, who was Phryne's defender and lover, threw off her clothes to arouse the judges and change their verdict. The judges acquitted thereafter.
There's different versions of the story, and the historical truth has been brought into doubt aplenty, however that's what's being depicted in that painting. https://en.wikipedia.org/wiki/Phryne#Trial