Volumes of CO2 correspond to the level of carbonation of the beer (over a few weeks assuming you leave the beer alone). Very carbonated beer has many volumes of CO2, lightly carbonated beer has less. See the key at the bottom of the chart which explains the color coding of the CO2 volumes.
To read the chart, find your temp on the left (the temp of your kegerator--38*F for me). Then, determine how carbonated you like your beer (I like mine more than most at, say, 2.9-3.0). So, you find your temp, look across the row to find where you want the CO2 volumes to be (in my case 3.03), and then look up--that tells you the PSI you should use to acheive that level of CO2 volumes at that temperature (in my example, 17 PSI).
The chart can work in reverse too: know your PSI and your temp, and where they intersect will tell you your CO2 volumes. Theoretically you could also use the chart to determine what temperature to set your kegerator if you know your PSI and volumes of CO2 you want to acheive.