If something is heliotropic, it turns towards the sun, like the plant in your house that seems to bend toward your window.
Heliotropic generally describes plants. It refers to the way that some plants change position throughout the day to follow the motion of the sun. In the morning, a daisy might face east; in the evening, it might face west. Helio comes from the Greek word hḗlios, which means "sun," and -tropic which also comes from Greek and means "turned toward." It's pretty straightforward!