Whenever you see the word "critical" in the name of a movement in the social sciences, you are looking at a theory that is, at its heart, Marxist. This is the case with Critical Criminology. This is a perspective that is centered around the idea that crime and justice are social constructs that are used to enhance the power of the ruling classes.
For example, to critical criminologists, reforms that seem to be progressive and positive (like actions aganst police brutality) are really just attempts by the dominant class to camouflage and conceal the true brutality of the whole system.
So, the central theme of this perspective is the idea that crime and justice are ideas that are used to perpetuate the power relationships in our society.