What does he mean by "line in the sand"?

Native speaker’s answer
Rebecca
By "line in the sand", he means the "limit" to what one will do. A point he won't go beyond. Jake's okay with "flipping off" his teacher, but he's not okay with lying. He won't lie- this is his line in the sand. Ex: It's okay if your room is a bit messy, but my line in the sand is leaving dirty dishes in the sink. Please wash them! Ex: This is my line in the sand. I will not run a marathon.