What does x in "Miscanthus × giganteus" name stand for?
It means that the plant is a hybrid. Having it in the middle means that it is between species of the same genus, else the × (it's not the letter x but the multiplication symbol ×) would have been in front. 

It is very commonly used in taxonomic nomenclature, namely every time you have a hybrid. This means you'll usually see it with plants, as animals very rarely interbreed. 

Wikipedia says that your plant is a hybrid from Miscanthus sinensis and Miscanthus sacchariflorus. It is an easy place to look up most hybrids you might encounter.