A port number is a numerical identifier used in computer networking to specify a particular process or service on a device. It helps direct incoming and outgoing data packets to the correct application. Port numbers range from 0 to 65535, with certain ranges designated for specific protocols, such as HTTP using port 80 and HTTPS using port 443. When a device communicates over the internet, it uses both an IP address and a port number to establish a connection. The combination of these two elements ensures that data reaches the right destination, allowing multiple services to run simultaneously on a single device without interference.