"Red line" warnings from the Obama administration – what do they mean?

The proverbial red line.

It's back in the news this week, as President Obama used the phrase when discussing Syria's stockpiles of chemical weapons. If Assad begins using them or even moving them around, in the President's words, it would be a red line for the United States.

We heard a lot about "red lines" when concerns over Iran's nuclear program spiked earlier this year. But are these red line warnings just talk?

Take these chemical weapons in Syria. The risk is serious and real. According to CIA reports, Syria has one of the most advanced chemical weapons programs in the Middle East. If they fell into the hands of Al Qaeda, it would create an incredibly dangerous situation.

OutFront tonight are Eric Schmitt, a reporter with The New York Times and the author of "Counterstrike: the Untold Story of America's Secret Campaign Against Al Qaeda," and Fran Townsend, a CNN homeland security contributor.