This is the 11727th most frequent Korean word.
낮지만
"낮지만" means "although/though it is low."
Here, '낮지만' is used as a contrast connector, highlighting the opposing qualities of being 'low' and 'wide'.
이 빌딩은 낮지만 넓어요.
This building is low but wide.
In this context, '낮지만' introduces the contrast between 'low grades' and the positive effort he is putting in.
그의 성적은 낮지만 노력하고 있어요.
His grades are low but he is making an effort.
The use of '낮지만' here contrasts the affordable price ('low') with the high quality ('superb').