The Text
The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States. They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place.
No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been encreased during such time; and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office.
The Meaning
Members of Congress are to be paid for their work from the U.S. Treasury. Amendment XXVII prohibits members from raising their salaries in the current session, so congressional votes on pay increases do not take effect until the next session of Congress.
Article I, Section 6, also protects legislators from arrests in civil lawsuits while they are in session, but they may be arrested in criminal matters. To prevent prosecutors and others from using the courts to intimidate a legislator because they do not like his or her views, legislators are granted immunity from criminal prosecution and civil lawsuits for the things they say and the work they do as legislators.
To ensure the separation of powers among the legislative, judicial and executive branches of government, Article I, Section 6, prohibits a senator or representative from holding any other federal office during his or her service in Congress.