“Draw a / the line", as a phrase, it means to set the limit of what you are willing to do. 限制克制自己去做自己喜欢或者想做的事情。歌词" drew a line for you” 意思是 为了你我可以放弃做自己原来想做的事自己的喜好。例如:A: Do you want to go out dancing on Thursday?
B: No, I have to draw the line. I can’t go out during the week anymore, because I’m too tired the next day and a bunch of works are waiting for me to finish. B很想出去玩,但这周实在是不能出去混了,因为第二天会很累,工作太多了。