Imagine a (tribal) leader in a desert area where sand is (everywhere). There are no trees, rivers or mountains, nothing to look at and say, "This is the (border) of our land, this is the place that we will (fight)to defend." Since there are no natural landmarks to relate to, that (commander) then takes a stick and drags it along the ground, and in that sense "(draws)" a line in the sand, and says, "Here is the border of our land that we will fight to (defend). Beyond this line, no enemy must come." It is from that (exotic) image that we get this idiom meaning to define a (symbolic) point beyond which there is no further compromise. Whatever ideas or discussions are on one side of the line, that is okay, but beyond that point there can be no (further discussion). The "line in the sand" represents a symbolic border for (negotiations).
to be built on sand 是指某事物基础不稳，随时会失败的意思。
to bury one's head in the sand 就是「鸵鸟心态」、「逃避现实」的意思。
to take sand to the beach 是指做没有必要的事，也就是「多此一举」的意思。
相关阅读：
零基础英语培训
日常英语口语培训
英语口语进阶培训
