The president pro tempore is the senatorial official who serves as president of the Senate when the vice president is absent. President pro tempore means "president for a time."
In the United States, one of the vice president's official duties is to serve as president of the Senate, but sometimes, the vice president is busy with other duties and cannot preside over daily senatorial operations. At these times, the president pro tempore, or president pro tem, will fill in as president of the Senate until the vice president returns. Only elected and active senators can fill this role. Though this is technically an elected position voted on by members of the Senate, the president pro tempore is typically designated as a member of the majority party who has been in continuous service as a senator for longer than any of his or her party peers.