Quick summary: The Open Door policy open free trade with China for all the countries. The Americans came up with this policy and they didn’t take any money for war reparations, while other countries who stayed in China during the Boxer Rebellion did and offered opportunities to Chinese students to study at American universities, establishing a good relationship with China. The other European countries are satisfied since they can now all trade with China and left China (before they took over a lot of the Chinese land).
If you don’t know about the Boxers’ Rebellion, it was about Chinese people who were going against all of the foreign military in China and plotted against them by causing damages to foreigners’ land.