The HUAC stands for the House Un-American Activities Committee. This part of the federal government investigated and called in individuals to Congress to answer questions about their ties to the Communist party or the Soviet Union in general. These witnesses were often called in with no true evidence. This ability to call in individuals without pressing formal charges or without evidence is dangerous, as it violates the constitution. In this case, several people faced violation of their 6th amendment rights (rights of the accused). Without having these rights, the government can essentially call in anyone they want, leading to possible harassment.
Due to these Congressional sessions, individuals were "blacklisted.
A perfect example of this is a group known as the "Hollywood 10." This group of Hollywood producers/actors refused to answer probing questions asked by Congress and were arrested for contempt. This process ruined their careers, as no company would hire them after being blacklisted.
