What's Up with Specific Impulse?

Specific Impulse is a common measure of rocket engine efficiency, but remains mysterious. What controls the specific impulse an engine creates? Why are some engines so much better than others? And why does it have the unit seconds? Referenced Videos: Planning your Solar System Road Trip: https://www.youtube.com/watch?v=LT0yTIUkNwU Build your own nuclear space tug: https://www.youtube.com/watch?v=WHx-UgDMTlI