What does "bricks and mortar" mean?

Basic and essential, as in Matthew Arnold's essay (1865): “Margate, that bricks-and-mortar image of British Protestantism.” This phrase transfers essential building materials to other fundamental matters. It also may be used more literally to denote a building or buildings (whether or not made of bricks and mortar), as in The alumni prefer to see their donations in the form of bricks and mortar. [Mid-1800s]