The Federal Business Number is a unique identifier that is given by the federal government to registered businesses to simplify their dealings with the government and comply with federal programs. It should not be confused with business registration or business licensing, which is a provincial matter and varies depending on your province or territory.
You only need to register for a Business Number if you need a program account for programs run by the federal government. The most common program accounts a business may need are the GST/HST (except in Quebec), federal payroll deductions, corporate income tax, and import/export. There are other programs that apply to businesses in specific fields, so you should check with the Canada Revenue Agency if you are unsure whether you need to register.