Hi Luis, this idiom is used as a way of asking someone what they are thinking about. Here's an example : a couple is out on their first date. Suddenly the girl becomes very quiet. The guy then might say, "A penny for your thoughts?" This is an old fashioned term. Nowadays the guy might say, tell me what are you thinking about.

You say "a penny for your thoughts" when you want to know what someone is thinking, especially if the person seems extra quiet, disturbed or very deep in thought. It's just a statement trying to get them to talk about what's on their mind.

It means: If you tell me what you are thinking right now, I will give you a penny. It's an old phrase form a time when a penny was a lot of money. Of course,you don't get any money, it's just asking you to speak out.