A more thorough explanation:

Watkins v. United States (1957) is a case that went to the U.S. Supreme Court. The case was about whether Congress could investigate people for things that were not related to the reason they were called to testify. The Court said that Congress could not do this because it violated the Due Process Clause of the Fifth Amendment. This means that Congress cannot force people to answer questions that are not related to the reason they were called to testify.

For example, in the case, John Watkins was called to testify about whether he was a member of the Communist Party. He denied the allegations, but then the Committee asked him about other people he knew who might be communists. Watkins refused to answer these questions because he did not think the Committee had the right to ask them. The Committee then held him in contempt and he was sentenced to one year in prison and a $100 fine. The Supreme Court overturned his conviction because the Committee's questions were not related to the reason he was called to testify.

This case was important because it set limits on Congress's power to investigate people. It said that Congress could only ask questions that were related to the reason the person was called to testify. This protects people's rights and ensures that Congress does not abuse its power.