Final answer:
A president's statement about his interpretation of an act of Congress that he is signing into law is known as a signing statement.
Step-by-step explanation:
The statement that a president issues about his interpretation of an act of Congress that he is signing into law is best described as a signing statement. Signing statements allow the president to express his opinion on parts of the legislation, especially concerning how he believes it should be executed or if he thinks certain provisions may be unconstitutional or infringe on executive powers. Although these statements are not as powerful as a veto, they can influence how laws are implemented and signal potential points of contention between the executive branch and Congress.