The tic-tac-toe grid divides the images in to thirds both horizontally and vertically. There is this theory in photography, that the four intersections between the lines are..for lack of a better word.."points of power" in an image.
It's claimed that more tension, energy and interest in the photo can be created by aligning to these intersections. E.g the nearest eye in a portrait, the an animal in a field etc.
It is based on, Pythagoras' theory of the golden mean - also known as the golden ratio, golden number or divine proportion.
The color histogram is a representation of an image derived by counting the 'color' of each pixel. The idea was proposed by Michael Swain and Dana Ballard in 1991. They are built from images in various color spaces - in DSLRs the RGB space, but it applies to CMYK and any other.
What they tell us as photographers:
There are a lot of insight into an image that can be gleaned from a color histogram, however it's most often used to detect whether an image is over or under-exposed. Or rather, how many of the pixels in the R G or B channel that are.
The histogram represents all pixels in the image, so it tells us how many of the pixels are over- or underexposed.
If there is a tall spike in the extreme left of the histogram, it's an indication that there are areas of the image that is underexposed and if it's on the right, it might be overexposed.