A wavelength is the distance in between the repeating units of a wave, as measured from one point on a wave to the corresponding point in the next unit. For example, the distance from the top -- called the crest -- of one wave unit to the crest of the next is one wavelength. In physics notation, wavelength is often designated by the Greek letter lambda. Wavelength is inversely proportional to the frequency of a wave. In other words, the shorter the wavelength is, the more wave units will pass in a given amount of time.
A wave is simply energy that moves through a medium. Outside of the context of physics, ocean waves are an excellent example of how waves work. Except where the wave breaks, it is not so much the water that moves as it is energy in the water, which produces an up-and-down motion that is noticeable a short distance from the shore. Physicists study light and sound waves, as well as waves of other types of energy, and in this context, wavelength is an important factor to define and consider.
Light waves are present all around us, in a very large range of wavelengths. This range is known as the electromagnetic spectrum, a small portion of which is perceived by our eyes as visible light. Light from the sun actually consists of the entire electromagnetic spectrum. Whether a given type of light is visible to us depends on its wavelength. If a light wave has a wavelength of between 400 and 700 nanometers, it will be visible to the human eye.
On either side of this range are increasingly short and increasingly long wavelengths, respectively. X-rays have wavelengths that are so short that they can go through solid objects. At the other end, some radio waves have wavelengths of 1 mile (1.6 km) or more.
Sound is another form of energy that travels in waves. Sound waves are similar to light waves, in at least two ways: how we perceive them depends on their wavelength, and there are many wavelengths that are too short or too long for us to perceive. The difference is that we usually define sound in terms of the wave frequency, rather than the wavelength, but these two are closely related, as already discussed. A sound wave with a long wavelength will have a low frequency, and we hear these waves as low-pitched sounds. High-pitched sounds come from waves with a short wavelength, and therefore a high frequency.