what can be understood from the magnitude response plot of a fitler

Hello,

I am not familiar with the filters so want to know of what can be understood or analysed from the maginitude response of a filter.

I want to know of what can be known from this maginitude response of the desinged lowpass filter

Fc   = 3;   %Cutoff frequency
N    = 11;  % FIR filter order
Hf   = fdesign.lowpass('N,Fc',N,Fc,100);%sampling frequency of 100Hz
Hd=design(Hf);
fvtool(Hd);

The response is

Thanks.