Soil gives plants the nutrients they need to combine with water and light and produce food through photosynthesis. Since houseplants can't expand their root system like outdoor plants, they must rely on the soil in the pot to provide all the nutrients they need. They must also have a balanced soil that is not too wet and heavy, but that isn't too light either. Although it's relatively simple to make soil using garden soil, compost, peat, and perlite, it's probably more convenient for a novice to simply buy premade potting soil. Be careful though, because there can be a big difference in the quality of potting soils.