A more thorough explanation:

An internal-security act is a law that makes it illegal to engage in subversive activities aimed at overthrowing or disrupting the government. These laws are designed to protect the government from threats posed by organizations that seek to undermine its authority.

For example, in the United States, the Internal Security Act of 1950 was passed to combat the perceived threat of communism during the Cold War. The law required members of communist organizations to register with the government and made it illegal to advocate for the overthrow of the government.

However, many provisions of internal-security acts have been declared unconstitutional, as they can infringe on individuals' rights to free speech and association. In 1993, the Internal Security Act was repealed in the United States.

Overall, internal-security acts are controversial because they balance the need to protect the government from threats with the need to protect individuals' civil liberties.