When light hits an object, not all of the different wavelengths (or colours) that make up that light will be reflected from the surface of the object. The wavelengths that do get reflected are what gives that object its colour. For example, a green object looks green to us because that is the wavelength of light that it reflects. If no wavelengths are reflected, the object will look black. But if all of the wavelengths are reflected the object will look white.
However, most objects are much more complex than this simple example. They reflect in many wavelengths, and they reflect some percentage at each of these wavelengths (it's not just all or nothing). So, when you plot the percentage of light that is reflected at each wavelength, you get a squiggly line that we spectroscopists call a reflectance spectrum (spectrum is the singular form of spectra). The thing to note is that most of these squiggly spectra have a number of peaks and valleys. There is no one wavelength to establish colour. Instead, the colour is determined by the strongest reflectors.
The important thing about spectra, which the leaf example demonstrates quite nicely, is that these squiggly lines can be used to identify components that make up the object you are observing. In the case of the leaf, its spectrum can tell you (among other things), whether or not chlorophyll is present in the leaves - though our eyes can tell us that almost as readily.
Which brings us to the planetary portion of the story. Reflectance spectra can be used to tell us which minerals make up the rocks on planetary surfaces. Each mineral has a very distinct spectrum, which can be used by experts and highly specialized software to identify it, allowing us to study the composition of planetary surfaces with remotely sensed spectral data.
My next post will build on this concept and talk about how we use spectral data to study the surface of the Moon...