Visible light refers to the type of light waves that we can see with our own two eyes. In practical terms, these light waves appear to us as the colors of the rainbow (red, orange, yellow, green, blue, indigo and violet) depending on their wavelength. When we view all of the visible light wavelengths together, we see white light. When you separate the wavelengths of light apart from one another through a prism or by some other means, the different colors of light appear as a rainbow.
There are a number of different ways that visible light can be affected including reflection, refraction, diffraction and interference.  Scientists are able to study the way that light behaves to help understand the universe. In astronomy, the most obvious example of the use of visible light is the telescope. Telescopes allow us to view the light that is reflected off of objects in space (such as planets) as well as light that is generated by objects in space (such as stars) or absorbed by objects in space (such as black holes).
Telescopes are used to document and measure a variety of different information about celestial objects. Here are just a few of the ways that visible light can be used in astronomy:
Spectroscopy. This process involves dispersing the light that an object emits, reflects or absorbs into its separate wavelengths. By analyzing the resulting wavelengths of light, scientists are then able to determine what physical characteristics the object has. For instance, they may be able to determine how hot or cold it is, how much mass it has, what it is made from and its luminosity. 
Photometry. This process involves measuring the brightness of a star, nebula, planet, galaxy or another object in space. The resulting data can help astronomers determine how far away a particular object is, how old it is, its overall structure and other important information. 
Polarimetry. This process involves measuring the polarization state of an object's light.
This fantastic video from PBS provides an in-depth look at how light can help astronomers understand the universe: