http://www.history.com/topics/cold-war/huac

Basically this committee was created in ww2 to protect against the Nazis and spying. After WW2 the attention was turned to the Communists as the cold war began. There was such a fear of communism (red scare and McCarthyism) that this was created to assess people of unAmerican activities, largely spying or if they supported the Communist party which many Hollywood actors did, hence many actors were trialed including Ronald Reagan. Again the UnAmerican activities were just people reporting other people as suspicious, but also if a person supported a communist magazine of the Communist party.