A more thorough explanation:

A materialman is a person who supplies materials for construction or repair of a structure or vehicle. They are also known as material suppliers.

For example, if a contractor needs lumber to build a house, they would purchase it from a materialman. The materialman would then supply the lumber to the contractor.

Materialmen have the right to file a mechanic's lien if they are not paid for the materials they supplied. This means they can place a legal claim on the property until they are paid.