[change] Examples of waves
All of these waves are sums of sine waves.
[change] "Length" of a sine wave
Here is a picture of a sine wave, showing its "wavelength":
The sine wave has a pattern that repeats. The length of this repeating piece of the sine wave is called the wavelength. The wavelength can be found by measuring the length or distance between one peak of a sine wave and the next peak. The wavelength can be found in many other ways too.