A pixel is a group of three colored dots; red, green, and blue. By changing the intensity of these three colors, every color in existence can be produced.
If the intensity of all 3 colors was set at 0, you would see black. If red and green were set at 0, but blue was set at max, you would see blue. If all three colors were set at max, you would see white.
Cheaper LCD screens can actually have pixels become blocked, (generally by dust), which will cause them to appear as black dots. Pixels can also burn out, but this is not very common.