Answer:
The president pro tempore of the U.S. Senate is the highest-ranking elected member of the chamber but the second-highest ranking officer of the chamber. The president pro tempore presides over the chamber in the absence of the vice president, who is the highest-ranking officer in Congress' upper chamber. The current president pro tempore of the U.S. Senate is Republican Orrin Hatch of Utah.
Writes the Senate Historical Office:
"Election of a senator to the office of president pro tempore has always been considered one of the highest honors offered to a senator by the Senate as a body. That honor has been bestowed upon a colorful and significant group of senators during the past two centuries — men who stamped their imprint on the office and on their times."
The term "pro tempore" is Latin for "for a time" or "for the time being." The powers of the president pro tempore are spelled out in the U.S. Constitution.