Fries's Structure of English, language teachers have described English words as falling into two broad types: those that belong in the dictionary like 'storm' and 'confabulate', called content or lexical words, and those that belong in the grammar like 'of' and 'the', called structure, function or grammatical words.
tl;dr: Structural Words are the meaningless little words in a sentence that make it flow or move from one important word to the next.
