Mortar is a masonry product composed of cement and sand. When water is mixed in with mortar, its binding element, the cement, is activated.
But we must distinguish mortar from "concrete," which acts in a similar way but which contains coarse aggregate which is bound together by the cement. Concrete can stand alone, while mortar more typically is used to hold brick, stone or similar materials together. For example, to construct a hardscape design feature such as a mortared stone wall, brick patio or a mortared wall around an artificial pond, it would be used to hold the bricks or stones together.