You tell each pixel where to go and what color to be by giving it two sets of values. For position, it needs (coords), called X and Y, usually written as (X,Y). The X coord is horizontal, the Y coord is vertical, and the numbers usually start at (0,0) in the upper-left corner.
For the pixel's color it needs the three RGB color values, written as (R,G,B). These go from 0 (no color) to 1 (full-on color). For instance, green is (0,1,0). Most paint programs use 0 to 255, but 0 to 1 is easier for Shaders to use. See also Texel.