What does it mean "This filter supports the following commands"?
Commands allow you to change the parameters during execution of the command. This is primarily useful when working with streaming media e.g. if the volume gets too loud during a broadcast, send a command to the volume filter to lower it.

To issue commands to filters which support it from outside, ffmpeg has to be compiled with libzmq support. See the docs for the zmq filter for details.

To issue commands from the terminal, press c

The 'command prompt' will be shown :

Enter command: &lt;target&gt;|all &lt;time&gt;|-1 &lt;command&gt;[ &lt;argument&gt;]


Enter a command as per the given format and press Enter.

e.g. overlay@logo -1 x 100