Short answer: Valve which permits blood to flow from the right atrium to the right ventricle is tricuspid valve
Long answer: The right side of the heart has lower pressure compare to the left side. There is a valve that support the lower pressure so the blood can move easily from the right atrium to the left ventricle. The valve is called tricuspid valve. It's called "Tri" because there are 3 flaps/leaflets on the valve.
For your information, the left side of the heart only has 2-leaflet valve, which make smaller circular opening compare to the 3-leaflet valve on the right side. That's because the left side has higher pressure than the right side.
