Meristems are areas in in a plant where the plant tissue is non-specialized; these areas can grow into new plant structures and formations.
An Apical meristem is found in the upper area of the plant, where sunlight contacts the surface. This area is where the primary growth of the plant takes place, providing new stems and leaves to gather sunlight and spread the plant's surface area. As the new stems grow, their tissue becomes specialized and the apical area moves upwards.
A Lateral meristem is found in the body of the plant, or the stem proper: these are also non-specialized tissues that form new girth for the plant, giving its structure strength and allowing more water to be pumped to the leaves by the roots. As the plant grows wider, the outer skin is replaced by new growth, while the inner skin either continues to grow or becomes hard and non-functional, as in cellulose -- tree wood. Lateral meristems grow the plant "laterally," or wider, while Apical meristems grow the plant "apically," or taller.