optimization in hfss

Can you explain further what you mean when you say optimization?

Changing any parametr.

S parameters are structure dependent so if you will change any parameter of structure they will change depending on the sensitivity.

Karen1992 yes it will change if u change any parameters

Whot is it optimization?

Sorry but your word optimization is very broad .
S parameters define the behavior of RF Structure like for two port network S11 and S22 define the reflection at port 1 and port 2 and S12 and S21 define the transmission. All of these parameters are highly frequency dependent and frequency of any RF structure is dictated by parameters of structure.
So if you change any parameter of your structure your S Matrix will definitely change.
Optimization has some definition that what you are trying to optimize and at what cost.