줄다 is something decreasing — money, portions, numbers: 교통사고가 크게 줄었다고 합니다 "traffic accidents have fallen sharply", 먹는 양이 왜 이렇게 줄었냐 "why has how much you eat dropped like this?". The longer 줄어들다 means the same and often suggests it happening gradually: 야속하게 줄어만 가는 회비, the kitty draining away. Its opposite is 늘다, and the transitive versions — a person doing the reducing — are 줄이다 and 늘리다.