Question

what is the cocept of gain bandwidth products

Answer 1

Every system(device) has this property. Gain*band-width is constant.
Let there be a OP-AMP with gain-bandwidth product=100Hz(
.
Initially let its gain be 10. Under this condition, your OP-AMP will work as amplifier of gain 10 only till 100/10( gain-bandwidth product/gain)=10Hz. After this frequency, gain will decrease from 10

Now let its gain be 50. Under this condition, your OP-AMP will work as amplifier of gain 50 only till 100/50( gain-bandwidth product/gain)=2Hz. After this frequency, gain will decrease from 50

and it goes.

This gain band width product is NEARLY a constant. It some times depends on the temperature at which the device operates, power supply given to the signal.

Please refer http://en.wikipedia.org/wiki/Gain%E2%80%93bandwidth_product

Answer 2

Generally your talking about devices with single pole gain roll-off
If you do a bode plot of gain vs frequency it's constant up to the 3dB point and then rolls off at 6dB/octave or 20 dB/decade. Now if you draw a gain line less then this it will intercept the original at a higher 3dB frequency, the product of the two lines gainX3dB frequency will be equal.