What's the S parameter in an Antenna
S-parameters are a way of expressing things with general waves instead of voltages and currents. It describes how much the waves are reflected or transmitted from/through a device.
With a device like an antenna, there‘s not only 1 but 4 S-parameters. The first one S_11 is also known as the the reflection coefficient.
If an antenna is tuned and matched, it is obvious that S_11 should be as small as possible at the operating frequency. (No wave should be reflected when feeding the antenna)
Maybe you don‘t want your antenna to be tuned to 2.4GHz, but to 2.44GHz, because with most radios like ZigBee or Bluetooth, you want to tune the antenna to the center of the frequency band.