All plants can sense the direction of the gravitational field | ELI5

All plants can sense the direction of the gravitational field and orientate themselves accordingly. This is called geotaxis. In mature plants, phototaxis (growing towards the light source) overrides the gravitational impulse for the stalk and leaves, but the roots - and the seed while it is underground - rely on gravity for orientation.
The mechanism is thought to be based on either the protoplasm (the living substance inside a cell) exerting a greater pressure on the cell walls at the bottom, or starch grains within the cells settling at the bottom. One or both of these cues influence the production of plant growth hormones that cause the plant to 'steer' as it grows.

Details for nerd 5 years old:
They have starchy components (amyloplasts) in some of their cells that are heavier than the cytoplasm, making them sediment at the bottom of the cell. A hormonal growth signal then emanates in the direction of the part of the cell where the amyloplasts accumulate (i.e. down).
You can see this for yourself by tipping over a fast-growing potted plant. The side of the stem facing down will grow faster, causing the shoot to gradually bend upright again. Plants use the same mechanism - uneven distribution of growth hormones in response to sensory input - when they grow toward light, etc.