A pixel is an object that contains a reference to the position and color of a single point in the instance's costume (a bitmap). The pixel's color can be reported or set using blocks from this category. The key to working with pixels is the pixels block, which returns a list of pixel objects, one for each pixel in the instance's costume.
It's possible to use the last two blocks in this category to directly get or set the color at a given x-y point in the costume, but for many image manipulation applications it is more convenient to work with a list of pixel objects.