Those sawtooth patterns at the back of the engine nacelles are called chevrons.
It’s an innovative technology, developed by Boeing and NASA, and they have only one purpose – noise reduction. The chevrons help smooth mixing of the air that’s gone through the engine, with the air that flows outside and around it. As this mixing of air becomes less turbulent, noise is reduced. And it works really well. So well that the 787 requires several hundred pounds less fuselage noise insulation.
It should make airport neighbors happier too, hopefully. Though I’m sure some of them are at their happiest when they have a chance to complain.
You don’t need to know all this to fly the 787, but it might be cool in case someone is asking.