The histogram on a DSLR will help you see the light distribution that the sensor is perceiving. Now you should always rely on what you see with your eyes, the histogram is only there to help you in difficult light conditions. You can read the following guide made by Ken Rockwell:

http://www.kenrockwell.com/tech/histograms.htm

 You can also look at the followingh resource to see what a good histogram should look like in different environments and conditions:

http://www.jclarksonphotography.com/articles/histogram.php