Offside tuning is the ability of your own com tuning panel to tune the other ones. The button lit is the one that it is both displaying and tuning.
thus you should normally see the 1 lit on Captain's side, 2 lit on F/o and three on the third box if fitted. You can tune Com 1 by reaching across or pushing the button 2 and tuning the one next to you.
It was a nice idea until much better systems came along......but it's the best we have on the 737....