CLEAN: Russia, Obama clinch deals to revive ties

The video discusses efforts to enhance nuclear security and prevent the spread of nuclear weapons. It highlights the importance of the US and Russia, as leading nuclear powers, in setting an example by reducing their nuclear arsenals. A joint understanding for a follow-on treaty to the START agreement is signed by Presidents Obama and Medvedev, aiming to cut nuclear warheads and delivery systems by up to one-third from current treaty limits.

Who are the two presidents mentioned in the text that signed the joint understanding?

What is the significance of the treaty mentioned in the text?