Sildenafil is a phosphodiesterase type 5 (PDE5) inhibitor. It helps by blocking the PDE5 enzyme, which is responsible for the breakdown of cyclic guanosine monophosphate (cGMP). By increasing cGMP levels, sildenafil promotes the relaxation of smooth muscles in the blood vessels of the penis, enhancing blood flow and facilitating an erection in response to sexual stimulation.