You may not think of salt and sugar as solids because when you see them in the kitchen they are such small particles. But each of these particles is as much a solid as a wooden table, a glass window, or a gold piece of jewelry. A solid is a collection of atoms or molecules that are held together so that, under constant conditions, they maintain a defined shape and size.
Pulse Width Modulation, or PWM, is a technique for getting analog results with digital means. Digital control is used to create a square wave, a signal switched between on and off.
The second PWM mode is called phase-correct PWM. In this mode, the timer counts from 0 to 255 and then back down to 0. The output turns off as the timer hits the output compare register value on the way up, and turns back on as the timer hits the output compare register value on the way down.