A business plan is a written document that describes how a business will achieve it's goals. It's a framework that will keep you on track as you go through the day to day operations of owning and operating your business.
Your business plan is a working document! You have to remember to revisit your plan quarterly or yearly as your business grows. Get into the habit of revisiting your business plan at least once a year! This will allow you to see what worked, what didn't work, where you want to go next, what you've accomplished, etc.
A business plan can be as short as one page or multiple pages depending upon the business sector. Please take a look at my course called "The Business Startup" for a detailed walk through of a business plan.
I've created a FREE template that starts you on your journey of starting and maintaining your business. This is not an end all be all template, but it will help you get started! If you aren't familiar with some of the terms outlined on the template, do you due dillegience and research the terms.