There is an article on Dpreview I believe about reading histograms but basically the left side is black and the right side is white center is midtones.

what you want to do is have even dispersion along this line without sharp spikes or large empty spaces once the graph begins as well as nothing to the extreme of either side. Best way I have found to get used to reading them aside form shooting and looking on camera is to open images in PS and open the levels adjustment to see the dispersion along the graph line.