Nationalism is the belief that your own country is better than all others. Sometimes nationalism makes people not want to work with other countries to solve shared problems.
It is important not to confuse nationalism with patriotism. Patriotism is a healthy pride in your country that brings about feelings of loyalty and a desire to help other citizens. Nationalism is the belief that your country is superior, without question or doubt. In some cases, nationalism can inspire people to break free of a foreign oppressor, as in the American Revolution, but nationalism can also lead a country to cut itself off from the rest of the world.