In this section the person running the program places a value into a variable while the program is running. That is, the program pauses and asks the user to enter a number and then continues running after the number has been entered. The entered number ends up being held as the value of a variable.
Here we are talking about a common situation: A program is running, and to complete its job it must get some information from the person running the program. This person is usually called the 'user'.
This is a type of 'input'. A form may be present that the user needs to fill out, or a dialog of some type might pop up asking the user for certain information, perhaps a name or number. The program usually waits, or pauses, until the user types in the reply to the request. After getting what it wants, the program proceeds to run, now using the newly entered information.
The important idea to get here is that this section is about getting information into a program while it is running, rather than having all the values hardcoded into the program before it is run.
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