The alpha value is used to combine the color value of the fragment being processed with that of the pixel already stored in the framebuffer. Blending occurs after your scene has been rasterized and converted to fragments, but just before the final pixels are drawn in the framebuffer
Without blending, each new fragment overwrites any existing color values in the framebuffer, as though the fragment is opaque.
Thus, you can use alpha blending to create a translucent fragment, one that lets some of the previously stored color value "show through."
For example, if you are viewing an object through green glass, the color you see is partly green from the glass and partly the color of the object. The percentage varies depending on the transmission properties of the glass: If the glass transmits 80 percent of the light that strikes it (that is, has an opacity of 20 percent), the color you see is a combination of 20 percent glass color and 80 percent of the color of the object behind it.
To have blending take effect, you need to enable/disable it: