A point of no return in your life? Things were no longer the same after it. It changed for better or worse. Because turning back is dangerous, physically impossible or difficult, or prohibitively expensive.
Its meaning comes from allusion to the crossing of the river Rubicon by Julius Caesar on his way back to Rome in early January 49 BC. His crossing of the river precipitated Caesar's civil war, which ultimately led to Caesar's becoming dictator for life.
poor mark 😢