A more thorough explanation:

The European Union (EU) is a group of European countries that work together to achieve full economic unity and eventually political union. The member nations agree to eliminate barriers to the free movement of capital, goods, and labor among themselves.

The EU was originally formed as the European Economic Community (EEC) by the Treaty of Rome in 1957. It was later renamed the European Community (EC) and became the European Union when the Maastricht Treaty on European Union took effect in November 1993.

For example, a citizen of France can work in Germany without needing a work permit because both countries are members of the EU. This is possible because the EU has eliminated barriers to the free movement of labor among its member nations.