ClearType is a technology Microsoft introduced in Windows to improve the visual display of fonts on a computer screen… specifically, on LCD monitors, but some improvement is used on the older CRT displays. Your computer screen is composed of finite sized elements called “pixels”. Everything you see displayed on your computer screen is composed of pixels. Before ClearType, text was displayed by approximating the drawing of textual characters to the pixels on your screen. Since each pixel was displayed a usually either just black or white for fonts, the edges of rounded fonts were blocky. Microsoft updated Windows with TrueType fonts in the 1990’s by drawing gray pixels beside the edges of curves on characters. This made them look smoother. But, the LCD display hardware is capable of 3 times the horizontal resolution than advertised, but at the expense of color.
Each pixel on an LCD monitor is composed of 3 smaller, physical components of red, green, and blue. If you look really closely at your monitor, and you have good eyes, or you use a magnifying glass, you can see that what appears as white from a distance is really individual red, green, and blue dots. These red, green, and blue dots can be brightened or darkened, individually, in 256 shades. This means red can be zero (off or black), 255 (bright red), or anywhere in-between. Same for the green and blue elements. With each of the 3 having their own shade at any given moment, that’s a combination of 256 * 256 * 256 = abut 16.7 million unique combinations for each pixel. These combinations produce all the colors you see on your monitor. Each combination is a unique color. Of course, in reality, they’re just shades of red, green, and blue, but from a distance, your eyes can’t tell them apart, so they merge and are perceived by your eyes and varying colors.
Since each pixel is really made of THREE sub-pixels (the red, green, and blue components), Windows can choose to display a pixel of a certain color to enable each red, green, and blue component separately, as individual pixels. This is what ClearType does. Instead of making a pixel a gray color (all three color components of equal brightness), it can increase red and decrease blue to brighten say, the LEFT side of the 3 components, or vice versa. Essentially, getting 3 times the resolution, at the expense of color correctness. This is sometimes noticeable to the user in the form of slightly colored edges on fonts that are supposed to be black and white, but the increase is resolution is usually worth it.