This applies to all subjects, I think. In a particular subject, you might be able to specify this a little more. You (should) learn how to think about problems, how to find enough material to understand a new problem, how to deal with lots of material, how to work to deadlines, … You learn how to persist on hard topics/problems, you learn how to think creatively, and in maths you certainly (should) learn how to think logically, how to make a proper logical argument, the importance of making the right assumptions and of knowing your assumptions, and so on. In Science I think you learn how to deal with data, what you can and can’t learn from data, etc.