Impedance matching of an amplifier circuit with a filter at the input and output
In your comments you specify that the operating frequency is 2.45GHz (correct me if I'm wrong). For a well matched system S11 and S22 are small linear values, meaning negative dB.
In your first plot your S11 and S22 are ~ -10dB at 2.45GHz, meaning 31.6% of an oncoming signal is reflected. Not too good.
After your match S11 is ~ -17.5dB and S22 is ~ -27.5dB at 2.45GHz. Now 13.3% of the wave reflects at the input, and only 4.2% reflects at the output. Overall a good improvement.