Section 1 states that the President's term ends on January 20th at noon, and the terms of the Senators and Representatives end on January 3rd at noon. There successors terms begin after they leave office.
No Mr. Obama! You can't pass that bill! Your term ends in an hour!
Section Two states that Congress will assemble once every year on January 3rd at noon.
Section Three says what happens when the president-elect dies. If this happens, the Vice President becomes president. If the president fails to qualify the Vice President becomes president until a knew president is chosen. If neither qualify, Congress can chose someone to temporally be president.
Sorry to see you die Mr. President, but I am president now!