what is RGB meter sensoring means ?

Have it straight from the horses mouth:

http://imaging.nikon.com/history/scenes/14/index.h...

You're question doesn't make much sense, RGB refers to the colour channels, exposure meters don't measure the colour channels they measure the Luminance.

There are white balance adjustments, but that is just a colour profile applied between the data and the output used to offset or enhance the colour of the light used, cameras don't meter colour.

Some cameras allow you to customise white balance to get rid of colour casts usually using a white piece of paper or a grey card, it uses the exposure meter to do it by measuring the grey tone and then writing a colour profile to make that paper sample a grey tone, but it uses the Luminance sensor to do it not the colour channels.

Chris

RGB stands for red, green and black - it is a combination of these colours that makes up the picture on a computer screen as well as if you print out your own photos or if you are entering a competition they usually require it. Professional printers use CYMK again the letters stand for colours that are combined - c stands for cyan, y is yellow, m is magenta and k is for key(black). I hope this is what you wanted to know.