His jokes are usually funny, but he needs to learn where to draw the line.

draw 在这里是动词，意思是"划定、划出"。 the 是定冠词，特指某个界限。 line 的意思是"线、界限"。 draw the line 是一个固定短语，字面意思是"划出界限"，引申为"设定限度、适可而止"的意思。