the level of income people have when they are extremely poor

Most students are on/close to/below the breadline. 大多数学生都十分贫困。

a group of people waiting outside a particular building to be given food

You'll see breadlines outside many New York churches at lunchtime. 午饭时间可以看到许多纽约教堂外面都排着等候施舍食物的队伍。