A port number, frequently also designated as port, is a number from 1 to 65535. In combination with an IP address, it designates a communication end point. A socket is always connected with a port number. As sockets are used by the communication function blocks of TCP/UDP communication, and these again by a program, a port number identifies a program, a server, or a client running on a controller.
If you communicate with the <IP of a controller>:<Port number>, then you communicate with a program that has connected itself to this port number. (The program has configured its communication function block such that it connects to this port number.)