Good Answer from Sri Jaganathan dear Ganesh Ji,

Just as received power needs to be quantified in scientific way, instead of saying so and so milli micro nano picco, femto or otto watt, we say -100 dBm -90 dBm -80 dBm, so on and so forth for making the communication more precise with the unit of logarithm scale that puts multiplying or dviding things as simple as addition or deletion or plus or minus symbolically with reference to a milli Watt

Any power about that of milli Watt has to be a positive power

And dB is relative changes, it is like saying, to reach some one now, you need to move five floors above or below current level which is plus or minus dB