[SmG indexcardqueen]

Syntax is the way words and phrases are arranged. So like if you were trying to describe a ball that was blue and very large, you would say "the large blue ball" instead of " the blue large ball" or if you're talking about a person doing a thing it would be subject + verb + object receiving the action i.e. "Venus posted on the forum"

So I assume by syntax essay they are just expecting an essay where you are mindful of the order you write the words