Necessary properties are characteristics that an object or concept must possess to be what it is. For example, the necessary properties of a triangle include having three sides and three angles. Without these features, it cannot be classified as a triangle. In philosophy, necessary properties are often discussed in relation to identity and existence. For instance, the necessary properties of water include being composed of H2O molecules. If something lacks these properties, it cannot be considered water, highlighting the importance of necessary properties in defining and understanding different entities.