You want to write in order to share a story with the world. While it would be nice to put yourself out there and have your work be well-received by all who can get their hands on it, it’s more likely that some people will better suited to being your readers than others.
The group of people you intend your writing to be appealing to make up your audience, and it’s important that you write with them in mind. Sure, sometimes a story might grip you and you may need to figure this out later on, but this may require you to make significant changes to suit the people you plan on marketing the piece toward.
Your audience will influence your writing in many ways. You will need to use language that these individuals will be able to understand and will be comfortable with. You will need to decide on appropriate topics, and make sure that you are choosing material that will work with the individual’s knowledge level and preferences. For example, a book on astrophysics filled with long, complicated sentences that are peppered with curse words won’t likely sell very well if you are writing for toddlers.
You will want to consider what your audience wants to read about as well. If your target audience is eating up a ton of vampire novels, you need to analyze it a bit to determine if writing another one will help you bring in some readers or if they will be sick of this type of story by the time your work hits the shelves.
Get to know your audience by spending time with them when you can. Talk to them and get their opinions on the books they read. Let them read your work and evaluate it to test the waters a bit and see how things are received.