How will multiple connections connect to this port at the same time?

a bit confused here...
Please let me know, how to manage the following:
There is only one IP port, it is 8999, there will we on listener on this port.
How will multiple connections connect to this port at the same time?
Where are the threads handle, and where are the threads' execute procedure?
I mean the listener have to open separate threads for each connection and executes its own code...