This is the 6184th most frequent Korean word.
수학
Mathematics.
In this sentence, '수학' (mathematics) is used as the subject that the speaker is studying.
나는 수학을 공부하고 있어요.
I am studying mathematics.
Here, '수학 문제' (math problem) refers to specific problems in the subject of mathematics.
수학 문제를 풀 때는 집중해야 해요.
When solving math problems, one must concentrate.
In this case, '수학' (mathematics) is used to denote the field in which the subject excels.