The wavelength of a light wave (or any other wave) is the distance between two peaks of the wave. As it is a physical distance, it has units of length. It is hard to "visualize" light as having a physical wavelength, but it does. That is not to say you can hold a ruler up to it and see how far it is between peaks and valleys. But, it does mean that this will have many implications for how a chemist might try to use light in an experiment. For example, a key characteristic of waves is diffraction. This occurs when the wave interacts with a group of objects that are separated by a spacing that is similar to the wavelength. Many compounds have atoms that are separated by distances of 0.1 nm. Therefore we would expect they would diffract light with similar wavelengths. X-rays have these wavelengths and are regularly used by chemists to determine the distance between atoms in a compound with x-ray diffraction.
Below is a picture of a wave denoting the wavelength with the symbol we commonly use a Greek lower case lambda, λ.
