Diastole is the period of time when the heart relaxes after contraction. Ventricular diastole is when the ventricles are relaxing, while atrial diastole is when the atria are relaxing.
During ventricular diastole, the pressure in the (left and right) ventricles drops from the peak that it reaches in systole. When the pressure in the left ventricle drops to below the pressure in the left atrium, the mitral valve opens, and the left ventricle fills with blood that was accumulating in the left atrium.
