The coefficient of friction is a number that describes the friction between two objects. A combination like rubber on concrete would have a really high coefficient of friction, whereas a greased baby on a slip-n-slide would have a really low coefficient of friction.
There's more to it, though-- every pair of objects has two coefficients of friction, one for static friction, whch applies when the objects are at rest, and one for kinetic friction which applies when objects are in motion. The kinetic coefficient is always lower, which is why something can be stuck on an incline, but as soon as you give it a tiny push, it slides easily. We have mental coefficients of friction, too, and they react the same way.
Preparing for my trip to China last fall, I knew that my laptop battery wouldn't last for the entire length of the flight. Rather than being a champion and just read, I decded to download the first season of Breaking Bad to watch on my phone. Being the paragon of discipline that I am, I figured I'd watch the first half of the season on the flight over (after exhausting my computer battery with work, of course), and then watch the second half on the way back.
So I got on my flight to China and worked until my battery was dead. That was easy, because working on my laptop is what I do. I read for an hour or so on my Kindle and then decided to check out Breaking Bad. As everyone said-- it was great. I watched it for the rest of the flight.