The reference sideband is needed to compute either input referred noise or noise figure. If you only require the output noise, you can set the input source on the form to "none", and then you won't have to fill this in.
If you do want either of these quantities, then this is to allow you to define the relationship between the desired signal path from the input to the output so that you can get the right gain used to divide the output noise, or compute the amount of noise caused by the input port at the output.
The number specified is to define the frequency translation, so for a given output frequency Fout, the input frequency would be |k*PSSfund+Fout| - or you can use the select from list to pick the right frequency.
So for example you are simulating a switch cap filter where the input and output frequencies are the same, it would be 0. For a down converting mixer it might be -1 and so on.