Lymphatic system- is it open or closed? explain what it means

Lymph is the excess blood plasma after it is filtered from interstitial fluid, when the interstitial fluid is formed.  From the circulation system, after filtration, lymph is comes into to the Lymphatic system.
The Lymphatic system with its tubular vessels, is an open system. It conducts lymph between various parts of the body.  It provides a route for the excess interstitial fluid to be returned back to blood.

On the other hand, Our cardiovascular system - responsible for circulation of blood - is a closed system. Blood never leaves the veins, arteries, and capillaries.

The mitral valve is also called the bicuspid valve because it contains two leaflets or cusps. ... The tricuspid valve has three leaflets or cusps and is on the right side of the heart. It is between the right atrium and the right ventricle, and stops the backflow of blood between the two.