Inside your mind, what you're thinking about, your ideas, your opinion, maybe your suggestion. Usually about here and now. Weighing on your mind, pushing you down, making you worried and unhappy, or making you distracted, something that takes your attention away from here and now. A; You have a crazy look on your face. What's in your mind? B: I'm going to ask that girl to dance with me. A: So we can build this in only two years ... B: The question in my mind is how do we pay for it? A: Are you listening to me? B: It's hard to pay attention while I have that girl on my mind. A: Can I talk to you? I need some advice. B: Sure, what's on your mind?

"What's on your mind?" is asking someone how they're doing, usually when they seem unhappy. "What's in your mind?" is rarely said so it will sound like you're asking what is literally inside their mind.

"What's on your mind?" means "What are you thinking about?" or "What is troubling you?" I have never used "What's in your mind?" personally. If someone said it to me, I would take it to mean "What are you thinking about?" or "What is your opinion?"