In writing, "content" means the actual ideas and information that you write.
"Content" can be contrasted with "style", which means the way that you write. Style includes the words that you choose and the way that you organize your sentences. "Content" can also be contrasted with "mechanics", which means spelling, grammar, capitalization, etc.
Notice that "contents" (with an "s") has a different meaning. It means what's inside of something, like the contents of a book or the contents of someone's purse.