There are a lot of ways of measuring the performance of an antenna over a frequency band,(eg,impedance bandwidth,directivity bandwidth,gain bandwidth and so on),impedance bandwidth of the microstrip patch antenna is the band in which the reflection coefficient(S11) is less than -10dB.
well no offense intended but since you are asking for what is the significance of s11,i reckon that you have taken a dive straight into antennas without getting a formal understanding of applied electromagnetics and transmission lines.it is very important to know the basics before getting into the field of antennas and propagation.
Hopefully you will have it figured out,if not drop a line.
Regards
Sajid Mohammed.