Learn in the textbookInput/Output and Procedures & Functions
DefinitionInput/output (入出力) is the combined name for input (入力), where a program takes in data from outside, and output (出力), where it passes the processed result back out.
ExplanationReading from the keyboard or a file counts as input, while showing something on screen or writing to a file counts as output. If you picture a program running in the order input → process → output, it becomes easy to follow which data comes in and goes out at each point. Even when the processing is split into several parts, you can ask the same question of each part: what goes in and what comes out.
AnalogyIt works like a restaurant: the order slip comes in, the kitchen cooks, and the finished dish goes out to the table.
Memory hookThe short form is I/O: the I of Input and the O of Output joined by a slash, packing both directions, in and out, into one word.