A mod.ini is a file that allows you to use the content of other mods in your own without having to install your game in their folder. Say I wanted to use a ship from Inferno and one from the FS1 port in my campaign. I'd make up a mod.ini that allowed access to both folders and then simply plonk it down in the folder belonging to my mod.
Then if my ships.tbl (or tbm) pointed at a pof file in one of the other mods the game would treat it as if it was present in my mod.
Basically it means you add copy and paste this code into a new text file. Call it mod.ini and stick it in your mod's folder. Then (assuming that you have the FS1 Port installed to a folder called fs1port) you can instantly use the music from the port.
infotext = This campaign shows what if the Vega node never collapsed from the Shivan's point of view ;
website = http://www.hard-light.net/forums/index.php/topic,40028;
forum = http://www.hard-light.net/forums/index.php/topic,40028;
[multimod]
secondrylist = ,fs1port,mediavps;
Note that mod.ini files only work for FS2_open not FRED2_Open. But you can do the same thing with FRED too. Simply make a shortcut to FRED and add the line -mod fs1port,mediavps to the Target box