What is "draw a line in the sand"

vb. 1 (context intransitive English) To lay down a challenge; to provide a test of commitment (often one which carries a high risk) to a cause. 2 (context idiomatic English) To create a real or artificial boundary or distinction between (two places, people or things). 3 (context idiomatic English) To indicate the threshold or level above which something will become unacceptable or will provoke a response; to create a boundary and imply or declare that its crossing will provoke a (negative) response.