If you play a note on a musical instrument or listen to a song, you can notice how some instruments, such as a bass guitar, have low, sonorous tones and other instruments, such as a ukulele, create high, biting tones. If you play a note on a piano and then play another note to the right of this note, you can hear that the second note will have a higher pitch. Waves are a visual representation of this difference in pitch.
Sound, like light, travels in waves. You can plot sound waves on an "X-Y" axis, as in a graph. A note on a piano produces a sound wave, which arches up and down in a series of equal hills and valleys. The pitch of a note relates to how steep these hills and valleys on the sound wave are, and how close together they are. Closely stacked sound waves are "short" waves and waves with the hills and valleys more spread out are "long" waves. You plot sound waves in this way to show the difference in pitch created by different sound waves. Sound waves also travel outward in a circle from the source of the sound, in a similar way to the ripples created by a pebble dropped in a pool of water. The height of the individual ripples in a sound wave represents the pitch of the sound, which you can plot on the "X-Y" axis.
Long waves have slopes that are more gradual, and each arc is further apart than in short waves. Low-pitched instruments such as a cello produce long waves. A vibrating string that is loose will vibrate many times, and oscillate slowly between the lowest point and the highest point of the vibration. This produces a low-pitched sound. If you tighten the string, it will vibrate much more quickly and produce a higher-pitched sound. A loose string produces long waves and this is why a loose string produces a lower-pitched sound.
When a car speeds past you, the pitch of the sound the car produces rises as the car approaches and lowers as it moves away. The "Doppler Effect" explains this phenomenon. The long sound waves produced by the car on its approach take time to get to you, and the car produces new sound waves while the older waves are still on their way to your ears. All of the longer waves produced by the car's journey stack up as they reach you, causing a higher-pitched sound. When the car moves away, the sound waves again become longer and produce a lower pitch.