What does it mean to have a flashing security light on a 1997 Nissan Pathfinder?

A flashing CEL light means that the computer has detected a pattern of misfires that's serious enough to damage the catalytic converter (a $1,200 part). You should stop driving the vehicle and get it to a shop immediately. It will not get better on its own and continued driving will not only damage the cat converter, but plug it up to the point where you won't be able to drive the vehicle anymore.