Why is my port fortified and what does that mean?

Good question. Your port has been fortified because the fermentation needed to be stopped in order to get the winemaker's desired result. When a wine is classified as "fortified" that means that the winemaker has added brandy or some other spirit to the fermenting grape must (juice and bits of seeds, stems and skins). He does not do this on a whim. He does this to create a particular style of wine ... in our case, port. The brandy or spirit kills the yeast which had been busy consuming the grapes' natural sugars and therefore halts fermentation. The result is, hopefully, sweet, high-alcohol wine that tastes good after dinner.