Your monitor and your eyes have a nonlinear response to the images I'm trying to communicate to you. "Gamma" refers to the character of this nonlinearity. To see these images as they are intended to be seen you need to set your monitor so that the above grey ramp appears linear -- that is, as a perfectly straight, not curved, ramp from black to white. If the above ramp stays black too long, and comes up to white only near the end, the images will appear too dark. Conversely (and more likely), if it ramps up to white too fast these images will appear too light, washed out.
Please try to get this setting right! Without that, all the many months of work I've put into getting the colors and contrast "just so" will be lost to you.
Of course, you really should view these images in a darkened room, and be sure that a number of other variables are right... But getting your gamma set correctly is the first and perhaps most important step.