In an application form, "nature of business" is the type or kind of business you are in. For example:
Company: Acme Steelworks
Nature of Business: Producing steel pipes for irrigation projects
Many employers use applications as a way of standardizing the information they obtain from all job-seekers, including some things that you would not normally put on your resume.
To get tips and advice, check out a job-seeker's guide to successfully completing job applications at Quintcareers.com.