Because of the earth's gravity, the roots grow down and the stem grows up. The gravity is a stimulus. Geotropism is the plant's way of responding. The roots going downward are positive because they are going toward the stimuli. The stem is negative because it is moving away from the stimulus.
Plants grow at a slow rate toward their stimuli.
When the plant grows toward the sun its called phototropism. The stem is what grows toward the sunlight. Chemical compounds called auxins control tropisms. One kind of auxins make growth cells. If one side of the plant is lighted then the auxins move away from that side.