What is the role of reflection coefficient in any microstrip antenna

Hello,
I want to study about all the parameters of antenna so today my question is about the reflection coefficient of the Antenna.
What is the role of reflection coefficient of the patch Antenna?

That is part of antenna effieciency, which denotes the matching status of the antenna.

When using antenna in harmonic oscillator, and antenna designet at 2*F0. Good reflection coefficient S11 of antenna can increase output at second harmonic, because F0 is reflected back to the oscillator, and if it has a good phase, it can sum up somehow with fundamental.
Also you can think in this way: if it is not reflected, then it is probably radiated =)