The vorticity is a quantity defined in a relativistic fluid that describes how much a fluid element is rotating and is accelerating. By measuring the spin polarization of hadrons, it was found that the quark gluon plasma produced in heavy-ion collisions is the most "vorticous" fluid ever observed. More generally, this opens the possibility to study the physics of QCD matter using spin. Here I use statistical quantum field theory applied to a fluid in local thermodynamic equilibrium to show how to connect the average spin of a fermion with hydrodynamic quantities, and in particular with the thermal vorticity and the thermal shear. I show that the spin polarization of a Dirac particle induced by thermal vorticity is related to the gravitational (in medium) form factor related to spin-rotation coupling. For these reasons, as we are understanding the role of spin in hydrodynamics and in heavy-ion collisions, spin is becoming a promising tool to investigate the properties of QCD and whose applications are just begun to be explored.