Definition - What does Port Scanning mean?
Port scanning refers to the surveillance of computer ports, most often by hackers for malicious purposes. Hackers conduct port-scanning techniques in order to locate holes within specific computer ports. For an intruder, these weaknesses represent opportunities to gain access for an attack.
There are 65,535 ports in each IP address, and hackers may scan each and every one to find any that are not secure.
Techopedia explains Port Scanning
While port scanning can be conducted for legitimate computer security reasons, it is also considered an open-door hacking technique, which can easily be performed for malicious reasons when a specific computer or operating system is the target. Conducted in stealth mode or strobe, malicious port scanning is typically conducted on ports after the 1,024 mark because the ports prior to that are usually affiliated with more standard port services. The ports following that mark are more susceptible to malicious port scanning due to their availability for probes.