Social security agreements are international treaties between two countries or nations that coordinate their social security laws. When an employee or self-employed person works abroad temporarily, a social security agreement serves to avoid double insurance cover - i.e. simultaneous social security coverage in the home country and in the country of the temporary stay (see also “Ausstrahlung” and “Einstrahlung”). This applies to those classes of insurance that are included in the respective agreement. Germany has concluded social security agreements with approximately 50 countries (incl. EU Member States). If no agreement is in place with a certain country, the country is referred to as a “non-contracting country.”

Source: Wikipedia, edited (only available in German)