Turning on the cool visual effects in iTunes is easy to do. Start by playing a song in iTunes, then press Command+t (hold down the cmd key, then press the t key).
You can exit the visual effect by pressing Command+t again or by pressing the escape key (esc) in the top left corner of your keyboard.
Visual effects are also customizable. This cool feature changes the type of visual effect on your screen. Just press the letter m on your keyboard, and wait a few seconds to see the new effect.
See the difference here:
Standard playlist – No visual effects v.s. Visual effects activated