From LQWiki

1) In TCP/IP networks, ports are used for incoming and outgoing connections to/from services running on a given host or node. TCP and UDP are the two protocols that typically use this system. For instance, BIND's implementation of DNS runs as the named service and by default listens on UDP port 53 (zone transfers use TCP port 53). The HTTP protocol typically listens on TCP port 80.

2) The verb to port refers to the process of translating a computer program so that it can run in another environment (typically on a different platform). Taking the linux kernel as an example: this might mean that a bugfix for the 2.4 kernel is ported to the 2.6 version, or (more often) the kernel is made to work on an architecture previously not supported.

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Read lsof to learn what processes are making use of a (networking) port.