3db ?

Hi, what is meant by 3db and why we always concern by it in filters ?? like F3db ?

3dB represents a fall in voltage that would result in half the power being dissipated in a resistor.

For example a 1V p-p signal dropped by 3dB would be 0.707V which would cut the power by half. An increase of 3dB would double the power so 1V would become 1.414V.

It's not always power though, it could be decibel volts. Though not frequently used the decibel as a unit of measure has no meaning without a reference.