<p>Spin polarization and spin transport are common phenomena in many quantum systems. Relativistic spin hydrodynamics provides an effective low-energy framework to describe these processes in quantum many-body systems. The fundamental symmetry underlying relativistic spin hydrodynamics is angular momentum conservation, which naturally leads to inter-conversion between spin and orbital angular momenta. This inter-conversion is a key feature of relativistic spin hydrodynamics, which is closely related to entropy production and introduces ambiguity in the construction of constitutive relations. In this article, we present a pedagogical introduction of relativistic spin hydrodynamics. We demonstrate how to derive constitutive relations by applying local thermodynamic laws and explore several distinctive aspects of spin hydrodynamics. These include pseudo-gauge ambiguity, the behavior of the system in the presence of strong vorticity, and the challenges of modeling the freeze-out of spin in heavy-ion collisions. We also outline some future prospects for spin hydrodynamics.</p>