Input involves getting data from some source. It might sensors, keyboard or any source. You might have a program that gets user input via a keyboard and another one that gets data via a file or URL. For your program to be useful you must provide it some data to work on (from any source).
Once data is in your program you have to process it in a meaningful way to solve your problem. The processing may involve, doing some arithmetic, making decisions based on certain conditions and looping through some things and performing actions on them.
After the processing on the data you expect your program to give you some kind of feedback. That feedback is what output is. It might give the feedback by displaying something on your monitor or by performing another visible action like maybe shutting down a computer or moving the arm of a robot.The feedback you get is dependent on what you want to achieve.