Can you describe what a heart valve is?

Yes: The heart is complex. Blood needs to flow in one direction. To keep from mixing and to allow flow in one direction, the valve are in place. They open and close with each beat to achieve this.

Yes: Blood moving through the heart is more effecient if it only moves in one direction. Valves open and allow blood to pass through. Then they close and prevent blood from moving backward from where it came. The aortic and pulmonic valves open in systole (during contraction). The mitral and tricuspid valves open during diastole (when the heart is resting and filling).