At its core, a business plan helps you prove to yourself and others whether or not your business idea is worth pursuing. It’s the best way to take a step back, look at your idea holistically, and solve for issues years down the road before you start getting into the weeds.
Set yourself a reminder to review your business plan regularly. If you have a team in your business, holding brainstorming sessions is a good way to benefit from their knowledge - it ensures you are all on the same page and that you get their support.
A business plan is a formal statement of business goals, reasons they are attainable, and plans for reaching them. It may also contain background information about the organization or team attempting to reach those goals.
Start from the very beginning understanding that your business plan ought to be specific to your business needs and objectives. Every business ought to have a plan, but not every business needs a full formal plan with carefully crafted summaries and descriptions.