he's talking about writing styles.
for example there's the expository writing style, that is used when one exposes facts to inform the reader on a particular subject. Nonfiction books, college thesis, news articles are written in this style. Writing a thesis proves very difficult for most students. The linguistic aspects of students' difficulties in writing a thesis will include problems mainly with grammar, vocabulary, punctuation and spelling.This type of writing style can be difficult, mostly because universities don't teach students how to write this way. Then there are nonfiction writings who are more simple. It depends on the individual and what his passions are: if someone is passionate about clothing is more likely to be a good writer of fashion. Simply because he spent a lot of time reading about fashion.
then there's the descriptive type, which is typical of poetry and diary entries. it involves the extensive use of methaphors. a lot of people are bad at writing poetry or to pour one's soul into a diary. again it takes practice to write this way.
then there's the narrative type, which is the one that is used to write novels. not a lot of people can write novels, lets face it. in my ex classroom only a few could really succed in doing it. Writing a novel takes a lot of time, immagination, theory of mind, memory and organizational skills.
then there's the persuasive type: let's say a letter to recomandation, product reviews, stuff like that. those can be simple. the writer has to adjust the language to the audience, but complex language can be learned if that's a problem. again it takes practice to write.
a lot of people write like s**t. when i was in school some classmates kept failing at it, and took a lot of Ds or Fs. if you really want to write, you should read more, and practice what you learn. good luck.