When I don't understand what people are saying in Spanish to me, I usually just say, "Si, si."
It's not a good habit.
They could be saying things like:
"You are stupid, aren't you?"
"Do you know that you have food all over your face and you look like an idiot?"
"Can I have one thousand euros right now, please?"
Or, more likely, they could be saying nice things to me, like, "You know you are beautiful, right?"
Or, "When you smile my soul feels so alive. I just want to dance with you. Will you dance with me right now with me for eternity?"