An FXS port is where you can plug in a regular telephone - liken an FXS port to a normal phone socket on your wall (for example a regular BT line socket). Like a normal phone socket, an FXS port will ring the phone, and present dial-tone when you lift the phone off the hook.
An FXO port, is less commonly used. An FXO port is a 'lisening' port and goes in place of where you would normally connect a telephone - for example into a regular phone socket on the wall. You cannot plug a telephone into an FXO port. An FXO port does not general ring current or dialtone for a telephone - it expects to receive ring current and dialtone from a phone line.
It is vital that you understand the difference between FXO and FXS so that you order the correct port type for your Vigor3300V VoIP ports - FXS is the more commonly used port - that's the type which allows you to connect in a normal telephone. Please seek further advice if you are still unclear.