In simple terms a IP connection has around 65000 "ports" (different applications use different ports).
Usually a firewall or border router will block incoming traffic (by closing all ports incoming) but you may have a reason to "open" a port allowing traffic to enter your network - this can be done for either a specific IP address / IP address range or can be configured to allow any IP address access.
Even if a port is blocked (incoming), should an outward bound port be opened - incoming traffic will usually be able to enter this port (e.g. at home I have all incoming ports blocked - when I open my internet explorer, port80 is opened bidirectionally).
You can find a list of common ports & their usages here:
http://www.iana.org/assignments/port-numbers Hope this helps