Diastole is the phase of the cardiac cycle during which the heart muscles relax after contraction, allowing the chambers of the heart to fill with blood. It follows systole, the phase where the heart contracts to pump blood out. During diastole, both the atria and ventricles are in a relaxed state, facilitating blood flow from the veins into the heart. Proper diastolic function is crucial for maintaining efficient circulation and overall cardiovascular health.