Original: The President pro tempore (or; also referred to as President pro tem) is the second-highest-ranking official of the United States Senate and the highest-ranking Senator. The United States Constitution states that the Vice President of the United States is the President of the Senate and the highest-ranking official of the Senate even though he or she votes only in the case of a tie.
Simplified: President pro tempore of the United States Senate is the longest serving Senator from a major political party in the United States Senate.