Sustainable is a difficult word to define. In simple terms it means that something can be maintained at a certain rate or level indefinitely. In relation to the environment, sustainability means meeting our present needs in a way that does not compromise the ability of future generations to meet their needs.
Example: Solar energy from the sun is sustainable. As long as the sun keeps rising we have access to an indefinite source of energy. This is referred to as a renewable energy. On the other hand oil, gas and coal are not sustainable sources of energy as they will run out one day.