Okay kiddo, you know how your heart is like a pump that helps your body move blood and give it oxygen? Well, sometimes people use the phrase "heart in hand" to describe a feeling of intense emotion, usually love or affection.
It's like when you really, really care about someone and you feel like your heart might burst out of your chest because of how much you love them. When people say they have their heart in their hand, it means they are being very open and vulnerable with their feelings, holding nothing back. It's a way of saying "I care about you so much that I'm putting my heart on the line for you." It can be a scary thing to do, but it can also be very beautiful and meaningful.