That it was a revolution of the bourgeoisie (capitalist middle class) against feudalism and that it "failed" because the proletariat (working class) didn't rise up. Karl Marx studied the French Revolution extensively and it was a big influence on his theories.

The Marxist view of history is essentially progressive. They believe that society advances through stages: the rule of feudalism, the rule of the bourgeoisie (capitalism), the rule of the proletariat (socialism), and finally a stateless communist utopia. Countries like the Soviet Union believed they were in the "rule of the proletariat" stage and that capitalist countries like the U.S. were lagging behind in the "rule of the bourgeoisie" stage.

So basically, Marxists see the French Revolution as a progressive development, but one which stopped short before it progressed all the way to socialism or communism.