Diastolic filling refers to the phase of the cardiac cycle during which the ventricles relax and fill with blood from the atria, primarily influenced by ventricular compliance and atrial pressure. It is crucial for maintaining adequate stroke volume and cardiac output, especially in conditions affecting ventricular relaxation or compliance. Impaired diastolic filling, as seen in diastolic heart failure, can lead to elevated filling pressures and symptoms of congestion despite preserved systolic function.