Making laws
president pro tempore
The Senate's presiding officer when the Vice President is absent, which is nearly always. By custom the longest-serving senator of the majority party, and third in the line of succession.
In full
The Constitution makes the Vice President the President of the Senate but expects absence, so it provides for a president pro tempore. The office is largely ceremonial: the holder rarely presides in person, delegating the chair to junior senators in rotation. Its real weight is symbolic seniority, plus a place behind the Speaker in the presidential line of succession.
Also written as: president pro tem
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.