The main purpose of this legislation, known as the "Third World Immigration Moratorium Act," is to restrict entry into the United States for individuals from certain countries that are deemed to be of concern. This act aims to enhance national security by limiting immigration from these countries, which may be associated with terrorism, instability, or other security risks. The key provisions of the act include a prohibition on entry for nationals from a specified list of countries, which includes nations like Afghanistan, Iran, Libya, and Venezuela, among others.