Learn in the textbookThe Preparation Stage of an Attack — Reconnaissance
DefinitionPort scanning (ポートスキャン) means trying to communicate with a target computer's ports (numbers that identify points of entry for communication) one after another to find out which ones accept connections.
ExplanationBecause each service is identified by a port number, knowing which ports are open gives a good idea of which services, such as web or email, are running. From there, an attacker gets clues for finding software with known vulnerabilities (security weaknesses). When administrators run it against their own organization's devices, it also serves as a check for ports left open without need.
AnalogyIt is like going down the hall of an apartment building pressing every intercom button in order and noting which units answer.
Memory hookTo scan is to read through from one end to the other, the same word as the scanner that reads paper documents.