Did you get this in a question in maths?
If you did ,it probably meant contrapositive. It means that it switches the hypothesis and conclusion with the the negation on both statements. For example, "If it is raining, then the floor outside is wet" if contrapositive is used, "If the floor outside is not wet, then it is not raining"
If you did not get this in a math question, contra means against or oppose.