The main purpose of the SECURE STEM Act is to enhance national security by restricting certain visas for individuals from specific countries, particularly focusing on nationals from the People's Republic of China. The act aims to prevent potential threats to U.S. education and critical research, especially in science, technology, engineering, and mathematics (STEM) fields. Key provisions of the act include a prohibition on issuing visas to individuals from designated countries who are applying for specific nonimmigrant visa categories.