This question can be answered in many ways, since each movie have many themes and views have differences of opinions. Here are a few important themes.
First, one of the great themes of the movie is the idea of freedom. They are free from the expectations of society and the strictures that society places on women. We can say that they break the categories of thoughts of women. This can be especially seen in the character, Thelma, passive housewife, who finds freedom.
Second, they break gender expectations as well. They take matters into their own hand. They literally fight several men. These are the men that stand in their way: Harlan, Darryl, JD, Hal, the truck driver, and the state trooper. What makes these men symbolic is that they are stereotypical.
Finally, another important theme is the idea of womanly friendship. The two women find great camaraderie with each other. There is incredible female bonding.