Consider the program.
LET MILES = 45678.3 - 45149.6 LET GALLONS = 12.5 PRINT MILES / GALLONS END
Every time you run the program it will do exactly the same thing. The next time you fill up your gas tank again, you would have to change the program to calculate with the new values. The program is not very useful.
Most useful computer programs input data from various sources when they run. Input means data that comes from outside the program. A program that does this can work with new data each time it is run.
For us, most data will come from the user of a program typing on the keyboard. Some sources of data (like audio data) require special hardware. A piece of hardware that is a source of input to a computer program is called an input device.
Pac-man was once a popular game found in video arcades. The game was really a computer program controlling the picture on the video screen. Think of several types of input data that the program used.