reactance -,+
Despite the resistive match between antenna and transmitter/receiver a reactive match is also required (with opposite sign). Non-zero reactance increases SWR of the antenna. Positive value of reactance means that antenna is too long (electrically, it's a simplification, but rather good), such an antenna acts as inductance (with radiation resistance in series). On the other hand, a negative reactance means that antenna is too short, and acts as capacitance (with resistance in series again).
The performance impact of reactive component of impedance is higher SWR when antenna is not matched. However, if it results from unmatched antenna dimensions, it will result in degraded radiation pattern.