A fortified medium is one that has been amended with nutrients. Vic's Super Soil is an example of a complete fortified medium, that is, the grower does not feed the plants during the time they are in this medium. Just add water.
A partially fortified medium contains some of the nutrients the plant will need, but the grower must also feed the plants during their growth cycle. ~shabang~'s casting/perlite/vermiculite mix may be partially fortified with guano and kelp meal. The grower will still need to feed the plants, but on a reduced schedule.