Okay kiddo, so once upon a time, there was a law called the Taft-Hartley Act, which made it illegal for certain people to hold union jobs if they were also part of a group connected to the Communist party.
The American Communications Association (a group of people who worked in the communications industry) didn't like that law and they decided to challenge it in court.
They went to court and argued that the law was unfair and violated their rights to free speech and association.
The case ended up going all the way to the Supreme Court, which decided that while the government did have the power to regulate who could hold certain jobs, the law went too far in restricting people's ability to associate with political groups.
So in the end, the court sided with the American Communications Association and said that the Taft-Hartley Act was unconstitutional when it came to restricting political association.