vocabulary
There are more than 187,000 words in the 2020 Oxford English dictionary and that doesn't include slang! No matter how many words you already know, you can always learn more.
I try to focus on teaching real English vocabulary wherever possible, this means it will probably be English you wouldn't find in textbooks as it will be a mixture of real English, slang and colloquialisms.
I teach my students vocabulary which will help you sound more natural, which in turn helps you to sound more fluent.