We just learned about the Light Year.
If you are standing in the water in the ocean or a big lake,
you can watch a wave going by you, something like this:
(from: wikipedia - wave)
If you stood in one place and watched a lot of waves going by, it might look something like this:
And if you took a picture of a wave while it was standing still it might look like this:
(from: wikipedia sine)
If you measured the distance from the start of the up wave, to the end of the down wave that is called the wavelength.