Why In rectangular notation an impedance is written R + jX, where the real part is resistance and the imaginary part is reactance. The sign of the j term tells you the type: positive j means inductive reactance, negative j means capacitive reactance. So 50 - j25 ohms is 50 ohms of resistance in series with 25 ohms of capacitive reactance.
Watch out The choice naming 25 ohms resistance with 50 ohms of reactance simply swaps the two numbers; the real part always comes first and the number attached to j is always the reactance.
Real part first, j part second. Minus j = Capacitor (think 'C' for 'cellar', below the line).