It means that one side of the plants are not receiving enough light. The shaded side of a plant stem grows faster than the sunny side because sunlight breaks down plant growth hormones, called auxins. Auxin promotes cell (and stem) elongation. You may have observed this phenomenon on your plants outdoors as well. Plants growing in shade often are oriented to the sunny side of the border. To avoid one-sided growth on your houseplants, give them a quarter turn every week or so. That way the sunlight will uniformly reach all sides of the plant over the course of a month, and plants will grow upright.