Plant geotropism (or, more properly gravitropism) is the growth of a plant in response to gravity. Yes, it's just that simple, though different aspects of plant growth may not indicate this. You might be confused because of a combination of responses of a plant to, say, gravity and light, or gravity and constant wind. Now that you are aware of this, you can certainly imagine some other combinations. The result of geotropism is for the plant to grow straight upright unless it has to grow towards the light.