An apical bud is the primary growing point located at the top of the plant. It is the main growth area in most plants. It exhibits apical dominance, inhibiting the growth of lateral buds so that the plant may grow vertically.
Apical buds produce auxin, which is distributed throughout the plant through the phloem. Auxin prevents lateral buds from growing so that the plant can use all available energy on growing the apical bud. When a plant grows tall, it is able to reach sunlight so that it can undergo photosynthesis. According to Wikipedia, apical dominance is most likely adaptive because if a plant is able to out compete others in the vicinity, it has higher fitness.