Something is concrete if it is not abstract: it is both particular and an individual, and (hence?) occupies some space and time. To say that a physical object is concrete is to say, approximately, that it is a particular individual that is located at a particular place and time.
In construction Concrete is a building material made from sand or other small stones, water, and cement. After mixing the cement hydrates and eventually dries out into a stone. Concrete was invented in the Roman Empire, but the knowledge of its making was lost, to be reinvented only in modern times.