Whatever you write, your work falls into one or more genres. We all have certain genres we love, and other genres we don't enjoy so much. You don't necessarily choose your genre — instead, writers usually fall into their genres based on their interests and feelings.
On the plus side, this means you likely already know a lot about your genre. Whatever you're writing, you've almost certainly been reading books, watching movies, or living experiences that line up with your genre. If you write poetry, then you no doubt enjoy the rhythms of language and the tight lines of condensed artistry. If you write memoir, then you've almost certaintly seen some things in your life — and the raw emotions of that writing will fit the authenticity that readers expect from life writing.
On the downside, there can be a disconnect between our writing and the audience expectations for a given genre. For example, lets say you love science fiction, but you also enjoy a florid writing style that describes all the flowers and the buzzing insects and the colorful furnishings of a room. Traditional science fiction readers might want more details about the plasma thrusters and a little bit less about what the commander supped upon during his afternoon tea.
Fortunately for us, there's no rule that says you must follow the genre. Understanding your genre can give ideas for how a typical story might go, but every story lives and breathes on its own. You don't need to write a straight horror story, you don't have to limit your poems to a single page, and you don't have to skip the florid descriptions of the starship captain's tea party. Because believe it or not, there are science fiction novels written in the flowery tones of Victorian England — and like any work of writing, those stories have fans who love reading them.
This is where genres help us. By looking at your themes and your style, you can determine which readers are the most likely to appreciate your work. You might not know who your audience is just yet, but understanding your genre will help you find them.