Quality is how good something is. If the quality of a product is high then that means that it is fit for its purpose. If the quality of an item is low that means that the product may break easily or not work properly. When people say something is a quality product that means that the product is of good quality. Usually higher-quality products cost more money, because the materials used to make them are better, or the way they were made was more effective. In other words, things that are low quality are cheap.
Quality is understood differently by different people, depending on what they consider quality to be. Generally, though, it is how well a product is suited to the purpose it was made for.