Light can be understood as a combination of energy waves traveling outward from a source, or as small packets moving from that source at the speed of light (each peak in the traveling waves would correspond to a single packet of energy, a photon). Light always travels at the speed of light, altering only for the material through which it's moving (it goes slower in water, for instance), so a segment of a light beam with wave peaks more separated (a longer wavelength) would have fewer peaks absorbed by a surface (a lower frequency) in any given amount of time, and would hit that surface with less energy. This means short wavelength = high frequency = more energy, long wavelength = low frequency = less energy. The only reason that this is important is that sunlight contains a fairly wide range of energy frequencies, but only a few are absorbed and used by chlorophyll, the energy-capturing molecule of photosynthesis.
You can tell a few of the frequencies that are not absorbed by chlorophyll (and a few other light-absorbing molecules) by looking at a plant. That green you see is part of the reflected frequencies of light. For the most part, absorption of the other frequencies of light is used in an energy conversion process that "spits" electrons through a system from the "excited" chlorophyll molecules. Energy conversion can also involve reradiation - electrons on certain atoms will absorb a particular frequency, "jump" to the next electron level (which, with just one electron in it, will be unstable), then "jump" back, releasing the energy at a different frequency (because some has been used). Some radiation is called ionizing radiation because it can make electrons jump completely off their atoms.
Although land plants absorb a variety of light frequencies, all frequencies are not equally powerful or useful: while plants can absorb both red frequencies and purple frequencies, the purple have shorter wavelengths and carry more energy. This is one of the reasons why "plant lights" are distinctly purple.
It is not unusual for land plants to use molecular supplements to absorb some frequencies that chlorophyll can't, and feed more energy into the photosynthesis process; these pigments are commonly types of carotenoids. The colors of leaves in the autumn reveals the carotenoids that have always been there but have been covered by huge amount of chlorophyll.
Carotenoids can serve multiple roles: they can be photosynthetic aids, but they may also minimize light damage (animals use pigments, like the human tan-producing molecule melanin, for similar protection) or even function in fighting disease. Land plants may concentrate pigments, including carotenoids, in structure that need to stand out, such as the colors of flowers or mature fruits. These colors signal animals that a food bribe is available, and then the animals are used to carry pollen or seeds.
Ranges of wavelengths associated with different types of energy.
Graph showing the many wavelengths in the sunlight that hits the atmosphere, and the fraction of those that reach the surface.
The wavelengths absorbed by plant pigment molecules.
A page on photosynthetic pigments.
A page specifically on carotenoids.
How autumn leaves get their color.
Pigment use in flowers.