The "input" command is used to interactively prompt for an expression that will be used to define (or redefine) a variable. The prompt parameter is displayed, followed by the variable and an equals (=) sign. After the expression is entered, the equation is submitted, and the variable is evaluated if there is sufficient information to do so.
The "optional" keyword can be used to prevent prompting for a variable that already has a value. If used, only an undefined variable would elicit a prompt... Defined variables would silently skip the command.
The input command is especially useful in scripts. It allows the user to provide values (or not) for specific variables, so pre-defined calculations can be performed automatically. The script below is an example of such a dialog.
input "Enter force applied (in Newtons)" f
input "Enter mass (in kg.)" m
'The resultant acceleration is: