Terahertz technology shows great potential applications in imaging, sensing and security. As is well known, the conventional solid-state broadband terahertz sources rely primarily on the nonlinear optical crystals and photoconductive antennas. Therefore, one major challenge for the next generation of terahertz technology is to develop the high-efficient, ultra-broadband and low-cost terahertz sources. In recent years, much attention has been paid to the spintronic terahertz emitters made of the metallic magnetic heterostructures on a nanometer scale. In this paper, the underlying physical mechanisms associated with this type of terahertz emitter is discussed. They mainly include the ultrafast demagnetization and the spin-charge interconversion processes. In order to further improve the terahertz emission efficiency, three main aspects are considered: appropriate choice of the materials (including conditions of the sample growing), film thickness, and new structure design. In the end, a short conclusion and future perspective for this research direction are given briefly.