Mass nouns, also known as uncountable nouns, refer to substances or concepts that cannot be counted individually. They represent a whole or a quantity that is not easily divided into separate units. Examples include water, sand, and information. These nouns typically do not have a plural form and are used with singular verbs. In contrast to count nouns, which can be quantified (like apple or car), mass nouns require different quantifying expressions. For instance, one might say "a glass of water" or "a bag of rice" to specify an amount. Understanding mass nouns is essential for proper grammar and communication.