“The galaxy is a really huge, flat disk, with the spiral arm pattern in it. Stars nearer the core orbit the center of the galaxy faster than ones farther out, and so you might expect the arms to wind up over time. But they don’t. That led astronomers to think of them like density waves, like traffic jams with stars instead of cars. A traffic jam isn’t a solid thing; cars enter it on one side, slow down, make their way through it, and leave on the other side. Over time, the jam is still there, but the individual cars come and go. That’s a density wave.”
Phil Plait is good at this stuff.