-3db !!?? what is it?

Lets consider an example:
To get frequency upto 100Hz and to reject all other higher frequencies, what will you do?
You will use a low pass filter of cut-off frequency 100Hz?! right ?
So now at the input of this filter you send a random signal of power say P (in watts).
In ideal case, at the output of the filter, you will expect to get signal which consists only 0-100 Hz frequency signals of power P, implying that the filter succeeded in attenuating all signals of frequency greater than 100Hz to 0 power and retaining full power of all signals with frequency ranging from 0-100Hz. But this does not happen in practical case, the signal begins to attenuate gradually even before 100Hz and attenuates sharply after 100Hz. Infact, for 100Hz (cut-off frequency), the power of the output signal is half the power of signal at the input of the filter ie., P/2 watts.
We know,

Plot of Gain vs Frequency for a low pass filter

Power Gain = Output Power / Input Power
Power Gain in db = 10 log (Output Power / Input Power)

what did you except from your low pass filter !

You expected that for all frequencies from 0-100Hz;
Output Power = Input Power.
or in terms of Gain:
Gain in db = 10 log (1)
               = 0 db

what did you actually get from your low pass filter !

For 100 Hz frequency, you found:
Output Power = Input Power/2
or in terms of Gain: 
Gain in db = 10 log (1/2)
               = -3 db

Conclusion:
Saying, at cut-off frequency gain is -3db, is just another way of saying that for the cut-off frequency, power of signal at the output is reduced to half the power supplied at the input. 
(This statement is valid for all kinds and types of filter at cut-off frequency)

Happy Learning !

p.s: Hope this cleared your concept ! If not, do comment what you aren't still clear about. Will be glad to give my best try.