The glass part would normally be completely red, but since it has 75% transparency it largely shows the page's background through it, making it seem a lot less red: For example, the following window texture has an alpha value of 0.25 at its glass part and an alpha value of 0.0 at its corners. This alpha value tells us exactly which parts of the texture have transparency and by how much. The textures we've used so far all consisted of 3 color components: red, green and blue, but some textures also have an embedded alpha channel that contains an alpha value per texel. An alpha value of 0.5 tells us the object's color consist of 50% of its own color and 50% of the colors behind the object. An alpha value of 0.0 would result in the object having complete transparency. Up until this chapter, we've always kept this 4th component at a value of 1.0 giving the object 0.0 transparency.
The alpha color value is the 4th component of a color vector that you've probably seen quite often now. The amount of transparency of an object is defined by its color's alpha value. Transparent objects can be completely transparent (letting all colors through) or partially transparent (letting colors through, but also some of its own colors). Transparency thus allows us to see through objects. This is also where the name blending comes from, since we blend several pixel colors (from different objects) to a single color. A colored glass window is a transparent object the glass has a color of its own, but the resulting color contains the colors of all the objects behind the glass as well. Transparency is all about objects (or parts of them) not having a solid color, but having a combination of colors from the object itself and any other object behind it with varying intensity. Blending in OpenGL is commonly known as the technique to implement transparency within objects.