Did average people have any knowledge of actual communist theory/goals, or was it mostly just known as something to be feared?
I'm interested in any point in time during the Cold War.
As a general rule, no.
"Communism" in the Cold War was widely interchangeable with Soviet imperialism. Indeed strictly speaking banning or persecuting communism was in fact against the American Constitution since communism is a political viewpoint.
However, espionage for a foreign country is not protected by the constitution, and it was under this guise that anti-communists like McCarthy operated under.