A material noun is a word that refers to a type of substance, rather than to individual particles of that substance. In grammatical terms, a material noun is non-count, concrete, and usually common. It is called "material" because many nouns in this class are the materials that other items are made out from, such as cloth, metal, wheat, etc., although there are some exceptions, such as rain or soap. Since the majority of material nouns are substances, most are common rather than proper nouns.
One way that grammarians classify nouns is count versus non-count nouns. In English, a quick check to find out if something is a count or non-count noun is to put the word "a" in front of it; if it makes sense with the "a", then it is a count noun. A material noun is always non-count, meaning that it cannot be pluralized. Since one would not, for example, refer to "two muds" or "five tars" these are non-count nouns. Some material nouns, however, can be used as count nouns, such "wine." One might talk about "three different wines," but in that instance, "wine" means "type of wine" rather than the substance of wine itself, so it is no longer a material noun in that usage.
Non-count nouns can be further divided into concrete or abstract. Concrete nouns refer to objects that can be observed with the senses, such as zebras or coffee. Abstract nouns refer to intangible concepts, such as love or existentialism. Material nouns are concrete by definition since they refer to a substance. Even the noun "air," which seems like it might be an abstract material noun since it cannot usually be seen, is actually concrete because it can be felt. For instance, one can feel the difference between hot air and cold air.
One of the most basic distinctions grammarians make among types of nouns is common versus proper. Proper nouns are capitalized and refer to specific people or places, such as George Washington or Uganda, or even brand names. Nearly all material nouns are common nouns, since they are substances rather than people or places, but a few are brand names, such as Lycra® or Spandex®.