since i am beginner, i want to know what is blending ? and what is the use of blending
Blending is the procedure to blend the incomming fragment with the existing fragment. The incomming fragment is more or less a pixel to be drawn, and the existing fragment is the pixel already in the framebuffer.
Blending is used when drawing translucent objects. If you want to draw a tranclucent object, you want some part of the image already in the framebuffer (the background) to show through the object. For each pixel to draw, OpenGL reads the pixel already in the framebuffer, and blends it with the pixel to be drawn. The alphachannel can be used to determine how much of the framebuffer is to be blended with the object.
Lens flares can use blending aswell. For example, the sun is not larger than your thumbnail at approx 50 cm distance from your eyes. But you generally see a huge flare around it. This flare can be drawn using additive blending. The incomming fragment is simply added with the existing fragment.
check my site (url in profile) i uploaded an app that shows what opengl blending does.