Original: While the Constitution requires the President to take this oath upon entering into the office, the 25th Amendment states that, upon it being invoked "the Vice President shall immediately assume the powers and duties of the office as Acting President."
Simplified: The Acting President of the United States is a person who temporary acts as President, when President can not carry out his duties, but remained in office. This position was started under the 25th Amendment to the United States Constitution.