The Congress of the United States was established by Article One of the U.S. Constitution. According to this article, the U.S. Congress is bicameral and thus Congress has consisted of two houses known as the Upper and the Lower houses (the Senate and the House of Representatives). This Article provides the U.S. Congress with some specified powers and the ability to enact legislation "necessary and proper" to exercise those endowments. The legislative powers of Congress are enumerated in Section VIII of Article One.
To cause a source to be biased means that it is influenced by your opinion or another factor that is not factual. Bias can take away from the credibility because it is not completely true, as a source should be.