The document discusses the rules for entering mathematical expressions into calculators, apps, and software using the BASIC input format. It explains that expressions from books cannot be directly typed in, and covers the syntax and semantics of the BASIC format. The format uses symbols like +, -, *, /, and ^ for operations and follows the order of operations like PEMDAS. The document provides examples of syntax errors and discusses how semantics are the rules for interpreting the meaning of syntactically correct expressions.