S11 is refelected input power and s22 is reflected output power.
In magnitude, we can say input return loss and output return loss.
Explanation for your query with example:
Assuem your passband is 500MHz to 600MHz, Impedance of your syatem is 50 ohm
Case 1: If return loss of S11 is 20dB in the band of 500MHz-600MHz, then max power (approx) transfer from input to ouput and vice versa.
Case 2: If return loss is less than 20dB in the band of 500MHz-600MHz, then less power transfer from input to output than case 1 ans so on..
Note: Outband of band no return loss available.
If you have still doubt, tell me your device (like amplifier, filter and etc..) and I will tell you more information.