擅长一门学科 be good at a subject
A：Do you mean you're good at this subject?
你的意思是你很擅长这门学科吗？
B：Yes. I like history very much.
是的。我非常喜欢历史。
mean /mi?n/ v. 意思是；意味着；打算
精通一门学科 know a subject from A to Z
A：Can you help me with this mathematical problem?
你能帮我解答一下这道数学题吗？
B：Sorry, I don't think I can solve it. You can ask Alan for help. He knows this subject from A to Z.