Normally you can ignore the port setting. HotkeyNet chooses one by default, and it will work on most computer networks.
However it may be the case that a program on one of your computers is already using HotkeyNet's default port number. If so, HotkeyNet will tell you, and you will have to change the port number on all your copies of HotkeyNet. Just try different random numbers till you find one that works. If you pick an invalid number, HotkeyNet will tell you.
When you choose a new port number, you must be sure to enter the same one on all your copies of HotkeyNet, or else they won't be able to communicate with each other.
Each time you change the port number, click "Apply" and restart HotkeyNet to make your new settings take effect.