Hello Nosrej
The phase margin as a measurement of stability for the op-amp when it connected in a resistive f.b circuit, it define you how much you are far from the oscillation which make the circuit unstable, the more you far the more stability. if we know that the oscillation condition phase is 180 then it mean how much you are from 180.
we usually measure this difference at the unity gain frequency point which represent the worst case, it is comparable for the buffer connection to assure its stability if we connect the op-amp like a buffer.
the value of the phase margin should be >= 45, optimally 60, in more also