Ok, All parameters in configuration has a note description next to it. It should be very clear.
In fact we are planning to build a wiki for all sort of question asked in this forum.
if you have converter installed, then you will need it. Otherwise you need to switch
"add video mechanism (fetch link only)" to "yes"
If converter are installed, likely are you using a dedicated server, vps hosting plan or shared media hosting. if you are using the first two, you can use a command via ssh to find out ">which ffmpeg" otherwise you need to contact your hosting company for it.