There are tendons in your hand. They are like puppet strings that the muscles of your forearm use to control your hand and fingers. If you use your muscle to tighten them, then your hand will curl into a fist. If you use your other hand to press on the spots indicated in the picture, then you will also tighten those tendons. Some people think it's strange for your hand to fold into a fist without your brain doing it on purpose. It's not dangerous, and doesn't do very much aside from make your hand change shape.