It's just a posh word that means the way that BigG looks at words in general, not so much keywords but more key phrases.
For example we all know the the word "Book" typed in Google search will bring in millions of listings because it just sees the word book! "Book" could mean anything related to a book. Book Ends, Book Bindings, Book Shelf, Book Library etc etc etc.
Semantic keywords or key phrases (Not to be confused with related keywords) are things like "Book Library" and then using keywords that basically mean the same thing! For example, Where do you Borrow Books, Where to find Books to borrow etc etc etc. Related keywords could mean anything else but they do not have to describe what your searching for. For example, "Book Library" and related keywords would be things like Book Rentals, Book Seller, Buy Books etc etc etc.
Again those keywords could be confused with what you actually mean? Do they mean you are a Book Seller? You want to find a book seller? etc etc etc. The more you can help BigG understand what you actually mean with the keywords/key phrases used the better.
I hope that helps.