The topic of discussion for this Constitution Monday comes from Article I.6.1: "Except for treason, a felony, or a breach of the peace, Senators and members of the House of Representatives shall be immune from arrest when their respective houses are in session or when they are going to or returning from a session." In order for Senators and members of the House of Representatives to perform their duties properly, they must be free to function in their duties in Congress without the worry of petty lawsuits, etc.
In order to prevent a legislator in England from voting on an important matter, they would be arrested on some insignificant charge. In the early days of our nation, a person could be arrested for a civil matter and delayed by petty suits. The Founders put this provision into the Constitution in order to prevent any delays for the Senators or members of the House of Representatives in performing their Congressional duties. This provision does not however prevent the arrest of a member of Congress for criminal activities.